diff --git a/examples/m17n/win_ansi_charset.rb b/examples/m17n/win_ansi_charset.rb
index f7da2d7a9..ffde37566 100644
--- a/examples/m17n/win_ansi_charset.rb
+++ b/examples/m17n/win_ansi_charset.rb
@@ -31,14 +31,14 @@
     code = "%d." % index
     char = index.chr
 
-    width = 1000 * font.width_of(char, :size => FONT_SIZE) / FONT_SIZE
+    width = 1000 * width_of(char, :size => FONT_SIZE) / FONT_SIZE
     size = "%d" % width
 
     data = [code, nil, char, size, nil, name]
     dx = x
     fields.zip(data).each do |(total_width, align), field|
       if field
-        width = font.width_of(field, :size => FONT_SIZE)
+        width = width_of(field, :size => FONT_SIZE)
 
         case align
         when :left then offset = 0
diff --git a/lib/prawn/document/text.rb b/lib/prawn/document/text.rb
index 3952b1757..b01a4f8cd 100644
--- a/lib/prawn/document/text.rb
+++ b/lib/prawn/document/text.rb
@@ -138,7 +138,7 @@ def wrapped_text(text,options)
           lines.each_with_index do |e,i|         
             move_text_position(font.ascender)
                            
-            line_width = font.width_of(e, :kerning => options[:kerning])
+            line_width = width_of(e, :kerning => options[:kerning])
             case(options[:align]) 
             when :left
               x = @bounding_box.absolute_left
diff --git a/lib/prawn/document/text/wrapping.rb b/lib/prawn/document/text/wrapping.rb
index 42aa38dd2..b0dc05aa1 100644
--- a/lib/prawn/document/text/wrapping.rb
+++ b/lib/prawn/document/text/wrapping.rb
@@ -32,7 +32,7 @@ def naive_wrap(string, line_width, font_size, options = {})
             segments = line.scan(scan_pattern)
                                           
             segments.each do |segment|    
-              segment_width = font.width_of(segment, :size => font_size, :kerning => options[:kerning]) 
+              segment_width = width_of(segment, :size => font_size, :kerning => options[:kerning]) 
         
               if (accumulated_width + segment_width).round > line_width.round
                 output = "#{output.sub(/[ \t]*\n?(\n*)\z/, "\n\\1")}"
diff --git a/lib/prawn/font.rb b/lib/prawn/font.rb
index af1eda3bd..17b099538 100644
--- a/lib/prawn/font.rb
+++ b/lib/prawn/font.rb
@@ -169,6 +169,29 @@ def font_families
                              :normal       => "Helvetica" }
         })
     end
+
+    # Returns the width of the given string using the given font. If :size is not
+    # specified as one of the options, the string is measured using the current
+    # font size. You can also pass :kerning as an option to indicate whether
+    # kerning should be used when measuring the width (defaults to +false+).
+    #
+    # Note that the string _must_ be encoded properly for the font being used.
+    # For AFM fonts, this is WinAnsi. For TTF, make sure the font is encoded as
+    # UTF-8. You can use the Font#normalize_encoding method to make sure strings
+    # are in an encoding appropriate for the current font.
+    #--
+    # For the record, this method used to be a method of Font (and still delegates
+    # to width computations on Font). However, having the primary interface for
+    # calculating string widths exist on Font made it tricky to write extensions
+    # for Prawn in which widths are computed differently (e.g., taking formatting
+    # tags into account, or the like).
+    #
+    # By putting width_of here, on Document itself, extensions may easily override
+    # it and redefine the width calculation behavior.
+    #++
+    def width_of(string, options={})
+      font.compute_width_of(string, options)
+    end
   end
 
   # Provides font information and helper functions.
@@ -226,19 +249,6 @@ def inspect
       "#{self.class.name}< #{name}: #{size} >"
     end
 
-    # Returns the width of the given string using the given font. If :size is not
-    # specified as one of the options, the string is measured using the current
-    # font size. You can also pass :kerning as an option to indicate whether
-    # kerning should be used when measuring the width (defaults to +false+).
-    #
-    # Note that the string _must_ be encoded properly for the font being used.
-    # For AFM fonts, this is WinAnsi. For TTF, make sure the font is encoded as
-    # UTF-8. You can use the #normalize_encoding method to make sure strings
-    # are in an encoding appropriate for the font.
-    def width_of(string, options={})
-      raise NotImplementedError, "subclasses of Prawn::Font must implement #width_of"
-    end
-
     # Normalizes the encoding of the string to an encoding supported by the font.
     # The string is expected to be UTF-8 going in, and will be reencoded in-place
     # (the argument will be modified directly). The return value is not defined.
diff --git a/lib/prawn/font/afm.rb b/lib/prawn/font/afm.rb
index a278c90af..af593f4e5 100644
--- a/lib/prawn/font/afm.rb
+++ b/lib/prawn/font/afm.rb
@@ -53,7 +53,7 @@ def bbox
       #
       # String *must* be encoded as WinAnsi
       #
-      def width_of(string, options={})
+      def compute_width_of(string, options={})
         scale = (options[:size] || size) / 1000.0
 
         if options[:kerning]
diff --git a/lib/prawn/font/ttf.rb b/lib/prawn/font/ttf.rb
index 34b76ae65..6f7022546 100644
--- a/lib/prawn/font/ttf.rb
+++ b/lib/prawn/font/ttf.rb
@@ -23,7 +23,7 @@ def initialize(document, name, options={})
       end
 
       # +string+ must be UTF8-encoded.
-      def width_of(string, options={})
+      def compute_width_of(string, options={})
         scale = (options[:size] || size) / 1000.0
         if options[:kerning]
           kern(string).inject(0) do |s,r|
diff --git a/spec/font_spec.rb b/spec/font_spec.rb
index 2333c25ed..9661357c4 100644
--- a/spec/font_spec.rb
+++ b/spec/font_spec.rb
@@ -135,13 +135,13 @@ def page_should_not_include_font(font)
   end
   
   it "should calculate string width taking into account accented characters" do
-    @times.width_of(@iconv.iconv("é"), :size => 12).should == @times.width_of("e", :size => 12)
+    @times.compute_width_of(@iconv.iconv("é"), :size => 12).should == @times.compute_width_of("e", :size => 12)
   end
   
   it "should calculate string width taking into account kerning pairs" do
-    @times.width_of(@iconv.iconv("To"), :size => 12).should == 13.332
-    @times.width_of(@iconv.iconv("To"), :size => 12, :kerning => true).should == 12.372
-    @times.width_of(@iconv.iconv("Tö"), :size => 12, :kerning => true).should == 12.372
+    @times.compute_width_of(@iconv.iconv("To"), :size => 12).should == 13.332
+    @times.compute_width_of(@iconv.iconv("To"), :size => 12, :kerning => true).should == 12.372
+    @times.compute_width_of(@iconv.iconv("Tö"), :size => 12, :kerning => true).should == 12.372
   end
 
   it "should encode text without kerning by default" do
@@ -168,12 +168,12 @@ def page_should_not_include_font(font)
   end
   
   it "should calculate string width taking into account accented characters" do
-    @activa.width_of("é", :size => 12).should == @activa.width_of("e", :size => 12)
+    @activa.compute_width_of("é", :size => 12).should == @activa.compute_width_of("e", :size => 12)
   end
   
   it "should calculate string width taking into account kerning pairs" do
-    @activa.width_of("To", :size => 12).should == 15.228
-    @activa.width_of("To", :size => 12, :kerning => true).should == 12.996
+    @activa.compute_width_of("To", :size => 12).should == 15.228
+    @activa.compute_width_of("To", :size => 12, :kerning => true).should == 12.996
   end
   
   it "should encode text without kerning by default" do