diff --git a/.gitignore b/.gitignore
index 49a35da6..51e06d06 100644
--- a/.gitignore
+++ b/.gitignore
@@ -10,3 +10,4 @@ doc
 coverage
 .bundle
 /.byebug_history
+.ruby-version
diff --git a/README.md b/README.md
index 0e25169b..ccd63211 100644
--- a/README.md
+++ b/README.md
@@ -25,70 +25,92 @@ This is a [Ruby][] implementation of a [SPARQL][] client for [RDF.rb][].
 ## Examples
 
 ### Querying a remote SPARQL endpoint
-    require 'sparql/client'
 
-    sparql = SPARQL::Client.new("http://dbpedia.org/sparql")
+```ruby
+require 'sparql/client'
+sparql = SPARQL::Client.new("http://dbpedia.org/sparql")
+```
+### Querying a remote SPARQL endpoint with a specified default graph
 
-### Querying a `RDF::Repository` instance
+```ruby
+require 'sparql/client'
+sparql = SPARQL::Client.new("http://dbpedia.org/sparql", { :graph => "http://dbpedia.org" })
+```
 
-    require 'rdf/trig'
-    repository = RDF::Repository.load("http://example/dataset.trig")
 
-    sparql = SPARQL::Client.new(repository)
+### Querying a `RDF::Repository` instance
 
-### Executing a boolean query and outputting the result
+```ruby
+require 'rdf/trig'
+repository = RDF::Repository.load("http://example/dataset.trig")
+sparql = SPARQL::Client.new(repository)
+```
 
-    # ASK WHERE { ?s ?p ?o }
-    result = sparql.ask.whether([:s, :p, :o]).true?
+### Executing a boolean query and outputting the result
 
-    puts result.inspect   #=> true or false
+```ruby
+# ASK WHERE { ?s ?p ?o }
+result = sparql.ask.whether([:s, :p, :o]).true?
+puts result.inspect   #=> true or false
+```
 
 ### Executing a tuple query and iterating over the returned solutions
 
-    # SELECT * WHERE { ?s ?p ?o } OFFSET 100 LIMIT 10
-    query = sparql.select.where([:s, :p, :o]).offset(100).limit(10)
+```ruby
+# SELECT * WHERE { ?s ?p ?o } OFFSET 100 LIMIT 10
+query = sparql.select.where([:s, :p, :o]).offset(100).limit(10)
 
-    query.each_solution do |solution|
-      puts solution.inspect
-    end
+query.each_solution do |solution|
+  puts solution.inspect
+end
+```
 
 ### Executing a graph query and iterating over the returned statements
 
-    # CONSTRUCT { ?s ?p ?o } WHERE { ?s ?p ?o } LIMIT 10
-    query = sparql.construct([:s, :p, :o]).where([:s, :p, :o]).limit(10)
 
-    query.each_statement do |statement|
-      puts statement.inspect
-    end
+```ruby
+# CONSTRUCT { ?s ?p ?o } WHERE { ?s ?p ?o } LIMIT 10
+query = sparql.construct([:s, :p, :o]).where([:s, :p, :o]).limit(10)
+
+query.each_statement do |statement|
+  puts statement.inspect
+end
+```
 
 ### Executing an arbitrary textual SPARQL query string
 
-    result = sparql.query("ASK WHERE { ?s ?p ?o }")
+```ruby
+result = sparql.query("ASK WHERE { ?s ?p ?o }")
 
-    puts result.inspect   #=> true or false
+puts result.inspect   #=> true or false
+```
 
 ### Inserting data into a graph
 
-    # INSERT DATA { <http://example.org/jhacker> <http://xmlns.com/foaf/0.1/name> "J. Random Hacker" .}
-    data = RDF::Graph.new do |graph|
-      graph << [RDF::URI('http://example.org/jhacker'), RDF::Vocab::FOAF.name, "J. Random Hacker"]
-    end
-    sparql.insert_data(data)
+```ruby
+# INSERT DATA { <http://example.org/jhacker> <http://xmlns.com/foaf/0.1/name> "J. Random Hacker" .}
+data = RDF::Graph.new do |graph|
+  graph << [RDF::URI('http://example.org/jhacker'), RDF::Vocab::FOAF.name, "J. Random Hacker"]
+end
+sparql.insert_data(data)
+```
 
 ### Deleting data from a graph
 
-    # DELETE DATA { <http://example.org/jhacker> <http://xmlns.com/foaf/0.1/name> "J. Random Hacker" .}
-    data = RDF::Graph.new do |graph|
-      graph << [RDF::URI('http://example.org/jhacker'), RDF::Vocab::FOAF.name, "J. Random Hacker"]
-    end
-    sparql.delete_data(data)
+```ruby
+# DELETE DATA { <http://example.org/jhacker> <http://xmlns.com/foaf/0.1/name> "J. Random Hacker" .}
+data = RDF::Graph.new do |graph|
+  graph << [RDF::URI('http://example.org/jhacker'), RDF::Vocab::FOAF.name, "J. Random Hacker"]
+end
+sparql.delete_data(data)
+```
 
 ## Documentation
 
-* {SPARQL::Client}
-  * {SPARQL::Client::Query}
-  * {SPARQL::Client::Repository}
-  * {SPARQL::Client::Update}
+* [SPARQL::Client](https://www.rubydoc.info/github/ruby-rdf/sparql-client/SPARQL/Client)
+  * [SPARQL::Client::Query](https://www.rubydoc.info/github/ruby-rdf/sparql-client/SPARQL/Client/Query)
+  * [SPARQL::Client::Repository](https://www.rubydoc.info/github/ruby-rdf/sparql-client/SPARQL/Client/Repository)
+  * [SPARQL::Client::Update](https://www.rubydoc.info/github/ruby-rdf/sparql-client/SPARQL/Client/Update)
 
 ## Dependencies
 
diff --git a/VERSION b/VERSION
index 4a36342f..cb2b00e4 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-3.0.0
+3.0.1
diff --git a/lib/sparql/client.rb b/lib/sparql/client.rb
index 84e3233d..de8ffdc5 100644
--- a/lib/sparql/client.rb
+++ b/lib/sparql/client.rb
@@ -40,7 +40,7 @@ class ServerError < StandardError; end
       '*/*;q=0.1'
     ].join(', ').freeze
     GRAPH_ALL  = (
-      RDF::Format.content_types.keys + 
+      RDF::Format.content_types.keys +
       ['*/*;q=0.1']
     ).join(', ').freeze
 
@@ -543,7 +543,7 @@ def parse_rdf_serialization(response, options = {})
       if reader = RDF::Reader.for(options)
         reader.new(response.body)
       else
-        raise RDF::ReaderError, "no suitable rdf reader was found."
+        raise RDF::ReaderError, "no RDF reader was found for #{options}."
       end
     end
 
@@ -701,8 +701,8 @@ def request(query, headers = {}, &block)
         if response.kind_of? Net::HTTPRedirection
           response = @http.request(::URI.parse(response['location']), request)
         else
-          return block_given? ? block.call(response) : response 
-        end          
+          return block_given? ? block.call(response) : response
+        end
       end
       raise ServerError, "Infinite redirect at #{url}. Redirected more than 10 times."
     end
@@ -726,6 +726,7 @@ def request_method(query)
     def make_get_request(query, headers = {})
       url = self.url.dup
       url.query_values = (url.query_values || {}).merge(:query => query.to_s)
+      set_url_default_graph url unless @options[:graph].nil?
       request = Net::HTTP::Get.new(url.request_uri, self.headers.merge(headers))
       request
     end
@@ -740,23 +741,56 @@ def make_get_request(query, headers = {})
     # @see    http://www.w3.org/TR/sparql11-protocol/#query-via-post-urlencoded
     def make_post_request(query, headers = {})
       if @alt_endpoint.nil?
-        endpoint = url.request_uri 
+        url = self.url.dup
+        set_url_default_graph url unless @options[:graph].nil?
+        endpoint = url.request_uri
       else
         endpoint = @alt_endpoint
       end
+
       request = Net::HTTP::Post.new(endpoint, self.headers.merge(headers))
       case (self.options[:protocol] || DEFAULT_PROTOCOL).to_s
         when '1.1'
           request['Content-Type'] = 'application/sparql-' + (@op || :query).to_s
           request.body = query.to_s
         when '1.0'
-          request.set_form_data((@op || :query) => query.to_s)
+          form_data = {(@op || :query) => query.to_s}
+          form_data.merge!(
+            {:'default-graph-uri' => @options[:graph]}
+          ) if !@options[:graph].nil? && (@op.eql? :query)
+          form_data.merge!(
+            {:'using-graph-uri' => @options[:graph]}
+          ) if !@options[:graph].nil? && (@op.eql? :update)
+          request.set_form_data(form_data)
         else
           raise ArgumentError, "unknown SPARQL protocol version: #{self.options[:protocol].inspect}"
       end
       request
     end
 
+    ##
+    # Setup url query parameter to use a specified default graph
+    #
+    # @see    https://www.w3.org/TR/sparql11-protocol/#query-operation
+    # @see    https://www.w3.org/TR/sparql11-protocol/#update-operation
+    def set_url_default_graph url
+      if @options[:graph].is_a? Array
+        graphs = @options[:graph].map {|graph|
+          CGI::escape(graph)
+        }
+      else
+        graphs = CGI::escape(@options[:graph])
+      end
+      case @op
+      when :query
+        url.query_values = (url.query_values || {})
+          .merge(:'default-graph-uri' => graphs)
+      when :update
+        url.query_values = (url.query_values || {})
+          .merge(:'using-graph-uri' => graphs)
+      end
+    end
+
     # A query element can be used as a component of a query. It may be initialized with a string, which is wrapped in an appropriate container depending on the type of QueryElement. Implements {#to_s} to property serialize when generating a SPARQL query.
     class QueryElement
       attr_reader :elements
diff --git a/lib/sparql/client/query.rb b/lib/sparql/client/query.rb
index 9a8b8fad..97610544 100644
--- a/lib/sparql/client/query.rb
+++ b/lib/sparql/client/query.rb
@@ -1,4 +1,4 @@
-module SPARQL; class Client
+class SPARQL::Client
   ##
   # A SPARQL query builder.
   #
@@ -366,16 +366,38 @@ def slice(start, length)
     end
 
     ##
-    # @example PREFIX dc: <http://purl.org/dc/elements/1.1/> PREFIX foaf: <http://xmlns.com/foaf/0.1/> SELECT * WHERE \{ ?s ?p ?o . \}
-    #   query.select.
-    #     prefix(dc: RDF::URI("http://purl.org/dc/elements/1.1/")).
-    #     prefix(foaf: RDF::URI("http://xmlns.com/foaf/0.1/")).
-    #     where([:s, :p, :o])
+    # @overload prefix(prefix: uri)
+    #   @example PREFIX dc: <http://purl.org/dc/elements/1.1/> PREFIX foaf: <http://xmlns.com/foaf/0.1/> SELECT * WHERE \{ ?s ?p ?o . \}
+    #     query.select.
+    #       prefix(dc: RDF::URI("http://purl.org/dc/elements/1.1/")).
+    #       prefix(foaf: RDF::URI("http://xmlns.com/foaf/0.1/")).
+    #       where([:s, :p, :o])
     #
-    # @return [Query]
+    #   @param [RDF::URI] uri
+    #   @param [Symbol, String] prefix
+    #   @return [Query]
+    #
+    # @overload prefix(string)
+    #   @example PREFIX dc: <http://purl.org/dc/elements/1.1/> PREFIX foaf: <http://xmlns.com/foaf/0.1/> SELECT * WHERE \{ ?s ?p ?o . \}
+    #     query.select.
+    #       prefix("dc: <http://purl.org/dc/elements/1.1/>").
+    #       prefix("foaf: <http://xmlns.com/foaf/0.1/>").
+    #       where([:s, :p, :o])
+    #
+    #   @param [string] string
+    #   @return [Query]
     # @see    http://www.w3.org/TR/sparql11-query/#prefNames
-    def prefix(string)
-      (options[:prefixes] ||= []) << string
+    def prefix(val)
+      options[:prefixes] ||= []
+      if val.kind_of? String
+        options[:prefixes] << val
+      elsif val.kind_of? Hash
+        val.each do |k, v|
+          options[:prefixes] << "#{k}: <#{v}>"
+        end
+      else
+        raise ArgumentError, "prefix must be a kind of String or a Hash"
+      end
       self
     end
 
@@ -823,4 +845,4 @@ def to_s
       end
     end
   end
-end; end
+end
diff --git a/lib/sparql/client/repository.rb b/lib/sparql/client/repository.rb
index dad6ddac..8bd07821 100644
--- a/lib/sparql/client/repository.rb
+++ b/lib/sparql/client/repository.rb
@@ -1,4 +1,4 @@
-module SPARQL; class Client
+class SPARQL::Client
   ##
   # A read-only repository view of a SPARQL endpoint.
   #
@@ -345,4 +345,4 @@ def insert_statement(statement)
     end
 
   end
-end; end
+end
diff --git a/lib/sparql/client/version.rb b/lib/sparql/client/version.rb
index 4f877bf6..3d16256c 100644
--- a/lib/sparql/client/version.rb
+++ b/lib/sparql/client/version.rb
@@ -1,4 +1,4 @@
-module SPARQL; class Client
+class SPARQL::Client
   module VERSION
     FILE = File.expand_path('../../../../VERSION', __FILE__)
     MAJOR, MINOR, TINY, EXTRA = File.read(FILE).chomp.split('.')
@@ -16,4 +16,4 @@ def self.to_str() STRING end
     # @return [Array(Integer, Integer, Integer)]
     def self.to_a() [MAJOR, MINOR, TINY] end
   end
-end; end
+end
diff --git a/sparql-client.gemspec b/sparql-client.gemspec
index dd209dd2..5e7b657c 100755
--- a/sparql-client.gemspec
+++ b/sparql-client.gemspec
@@ -20,12 +20,7 @@ Gem::Specification.new do |gem|
   gem.platform           = Gem::Platform::RUBY
   gem.files              = %w(AUTHORS CREDITS README.md UNLICENSE VERSION) + Dir.glob('lib/**/*.rb')
   gem.bindir             = %q(bin)
-  gem.executables        = %w()
-  gem.default_executable = gem.executables.first
   gem.require_paths      = %w(lib)
-  gem.extensions         = %w()
-  gem.test_files         = %w()
-  gem.has_rdoc           = false
 
   gem.required_ruby_version      = '>= 2.2.2'
   gem.requirements               = []
diff --git a/spec/client_spec.rb b/spec/client_spec.rb
index 001846e8..90368232 100644
--- a/spec/client_spec.rb
+++ b/spec/client_spec.rb
@@ -227,6 +227,51 @@ def response(header)
       end
     end
 
+    context "with multiple Graphs" do
+      let(:get_graph_client){ SPARQL::Client.new('http://data.linkedmdb.org/sparql', {:method => 'get', :graph => 'http://data.linkedmdb.org/graph1'}) }
+      let(:post_graph_client10){ SPARQL::Client.new('http://data.linkedmdb.org/sparql', {:method => 'post', :graph => 'http://data.linkedmdb.org/graph1', :protocol => '1.0'}) }
+      let(:post_graph_client11){ SPARQL::Client.new('http://data.linkedmdb.org/sparql', {:method => 'post', :graph => 'http://data.linkedmdb.org/graph1', :protocol => '1.1'}) }
+
+      it "should create 'query via GET' requests" do
+        WebMock.stub_request(:get, 'http://data.linkedmdb.org/sparql?default-graph-uri=http%3A%2F%2Fdata.linkedmdb.org%2Fgraph1&query=SELECT%20%3Fkb%20WHERE%20%7B%20%3Fkb%20%3Chttp%3A%2F%2Fdata.linkedmdb.org%2Fresource%2Fmovie%2Factor_name%3E%20%22Kevin%20Bacon%22%20.%20%7D').
+          to_return(:body => '{}', :status => 200, :headers => { 'Content-Type' => 'application/sparql-results+json'})
+        get_graph_client.query(select_query)
+        expect(WebMock).to have_requested(:get, "http://data.linkedmdb.org/sparql?default-graph-uri=http%3A%2F%2Fdata.linkedmdb.org%2Fgraph1&query=SELECT%20%3Fkb%20WHERE%20%7B%20%3Fkb%20%3Chttp%3A%2F%2Fdata.linkedmdb.org%2Fresource%2Fmovie%2Factor_name%3E%20%22Kevin%20Bacon%22%20.%20%7D")
+      end
+
+      it "should create 'query via URL-encoded Post' requests" do
+        WebMock.stub_request(:post, 'http://data.linkedmdb.org/sparql?default-graph-uri=http%3A%2F%2Fdata.linkedmdb.org%2Fgraph1').
+          to_return(:body => '{}', :status => 200, :headers => { 'Content-Type' => 'application/sparql-results+json'})
+        post_graph_client10.query(select_query)
+        expect(WebMock).to have_requested(:post, "http://data.linkedmdb.org/sparql?default-graph-uri=http%3A%2F%2Fdata.linkedmdb.org%2Fgraph1").
+          with(:body => "query=SELECT+%3Fkb+WHERE+%7B+%3Fkb+%3Chttp%3A%2F%2Fdata.linkedmdb.org%2Fresource%2Fmovie%2Factor_name%3E+%22Kevin+Bacon%22+.+%7D&default-graph-uri=http%3A%2F%2Fdata.linkedmdb.org%2Fgraph1")
+      end
+
+      it "should create 'query via Post directly' requests" do
+        WebMock.stub_request(:post, 'http://data.linkedmdb.org/sparql?default-graph-uri=http%3A%2F%2Fdata.linkedmdb.org%2Fgraph1').
+          to_return(:body => '{}', :status => 200, :headers => { 'Content-Type' => 'application/sparql-results+json'})
+        post_graph_client11.query(select_query)
+        expect(WebMock).to have_requested(:post, "http://data.linkedmdb.org/sparql?default-graph-uri=http%3A%2F%2Fdata.linkedmdb.org%2Fgraph1").
+          with(:body => select_query)
+      end
+
+      it "should create requests for 'update via URL-encoded POST'" do
+        WebMock.stub_request(:post, 'http://data.linkedmdb.org/sparql?using-graph-uri=http%3A%2F%2Fdata.linkedmdb.org%2Fgraph1').
+          to_return(:body => '{}', :status => 200)
+        post_graph_client10.update(update_query)
+        expect(WebMock).to have_requested(:post, "http://data.linkedmdb.org/sparql?using-graph-uri=http%3A%2F%2Fdata.linkedmdb.org%2Fgraph1").
+          with(:body => "update=DELETE+%7B%3Fs+%3Fp+%3Fo%7D+WHERE+%7B%7D&using-graph-uri=http%3A%2F%2Fdata.linkedmdb.org%2Fgraph1")
+      end
+
+      it "should create requests for 'update via POST directly'" do
+        WebMock.stub_request(:post, 'http://data.linkedmdb.org/sparql?using-graph-uri=http%3A%2F%2Fdata.linkedmdb.org%2Fgraph1').
+          to_return(:body => '{}', :status => 200)
+        post_graph_client11.update(update_query)
+        expect(WebMock).to have_requested(:post, "http://data.linkedmdb.org/sparql?using-graph-uri=http%3A%2F%2Fdata.linkedmdb.org%2Fgraph1").
+          with(:body => update_query)
+      end
+    end
+
     context "Error response" do
       {
         "bad request" => {status: 400, error: SPARQL::Client::MalformedQuery },
diff --git a/spec/query_spec.rb b/spec/query_spec.rb
index c2da28fc..98ad7645 100644
--- a/spec/query_spec.rb
+++ b/spec/query_spec.rb
@@ -182,9 +182,25 @@
       expect(subject.select.where([:s, :p, :o]).slice(100, 10).to_s).to eq "SELECT * WHERE { ?s ?p ?o . } OFFSET 100 LIMIT 10"
     end
 
-    it "should support PREFIX" do
+    it "should support string PREFIX" do
       prefixes = ["dc: <http://purl.org/dc/elements/1.1/>", "foaf: <http://xmlns.com/foaf/0.1/>"]
-      expect(subject.select.prefix(prefixes[0]).prefix(prefixes[1]).where([:s, :p, :o]).to_s).to eq "PREFIX #{prefixes[0]} PREFIX #{prefixes[1]} SELECT * WHERE { ?s ?p ?o . }"
+      expect(subject.select.prefix(prefixes[0]).prefix(prefixes[1]).where([:s, :p, :o]).to_s).to eq "PREFIX dc: <http://purl.org/dc/elements/1.1/> PREFIX foaf: <http://xmlns.com/foaf/0.1/> SELECT * WHERE { ?s ?p ?o . }"
+    end
+
+    it "should support hash PREFIX" do
+      prefixes = [{dc: RDF::URI("http://purl.org/dc/elements/1.1/")}, {foaf: RDF::URI("http://xmlns.com/foaf/0.1/")}]
+      expect(subject.select.prefix(prefixes[0]).prefix(prefixes[1]).where([:s, :p, :o]).to_s).to eq "PREFIX dc: <http://purl.org/dc/elements/1.1/> PREFIX foaf: <http://xmlns.com/foaf/0.1/> SELECT * WHERE { ?s ?p ?o . }"
+    end
+
+    it "should support multiple values in PREFIX hash" do
+      expect(subject.select.prefix(dc: RDF::URI("http://purl.org/dc/elements/1.1/"), foaf: RDF::URI("http://xmlns.com/foaf/0.1/")).where([:s, :p, :o]).to_s).to eq "PREFIX dc: <http://purl.org/dc/elements/1.1/> PREFIX foaf: <http://xmlns.com/foaf/0.1/> SELECT * WHERE { ?s ?p ?o . }"
+    end
+
+    it "should raise an ArgumentError for invalid PREFIX type" do
+      inavlid_prefix_types = [RDF::URI('missing prefix hash'), 0, []]
+      inavlid_prefix_types.each do |invalid_arg|
+        expect { subject.select.prefix(invalid_arg) }.to raise_error ArgumentError, "prefix must be a kind of String or a Hash"
+      end
     end
 
     it "should support OPTIONAL" do