diff --git a/docs/11-error-index.md b/docs/11-error-index.md
new file mode 100644
index 0000000..ec33b1a
--- /dev/null
+++ b/docs/11-error-index.md
@@ -0,0 +1,13 @@
+# Compiler Error Index
+
+Each diagnostic emitted from the compiler has a unique error number of the form
+`SCMXXX` where `XXX` is the error number. Error numbers are grouped into
+'categories' based on the phase in the compiler that emitted the diagnostic.
+
+Currently the diagnostic categories are:
+
+ * `1-9` - Legacy syntax errors.
+ * `10-19` - Parser and read.
+ * `20-29` - Library declaration and use.
+ * `30-39` - Binder diagnostics.
+ * `40-49` - Macro definition and expansion.
diff --git a/src/Feersum.CompilerServices/Binding/Binder.fs b/src/Feersum.CompilerServices/Binding/Binder.fs
index 23ee1ac..2b080dd 100644
--- a/src/Feersum.CompilerServices/Binding/Binder.fs
+++ b/src/Feersum.CompilerServices/Binding/Binder.fs
@@ -59,6 +59,11 @@ type BoundLiteral =
         | SyntaxConstant.Boolean b -> BoundLiteral.Boolean b
         | SyntaxConstant.Str s -> BoundLiteral.Str s
 
+/// Bound Datum Element
+///
+/// Represents a form or atom when used as a data element rather than as a
+/// program element. Used to hold the contents of quoted expressions, as well
+/// as the lements winthin a vector literal.
 and BoundDatum =
     | Compound of BoundDatum list
     | SelfEval of BoundLiteral
diff --git a/src/Feersum.CompilerServices/Diagnostics.fs b/src/Feersum.CompilerServices/Diagnostics.fs
index 86d16b6..8be423f 100644
--- a/src/Feersum.CompilerServices/Diagnostics.fs
+++ b/src/Feersum.CompilerServices/Diagnostics.fs
@@ -79,6 +79,9 @@ type Diagnostic =
           Message = message }
 
     /// Format the diagnostic for output.
+    ///
+    /// The output format shold conform to the [MSBuild "Canonical Error
+    /// Format"](https://github.com/dotnet/msbuild/blob/94c28cca4cdb22f2cac279e3fd8d86aa4d061848/src/Shared/CanonicalError.cs#L12-L51)
     override d.ToString() =
         let normaliseName (stream: string) =
             if Path.IsPathRooted(stream) then
@@ -91,18 +94,30 @@ type Diagnostic =
         | Point p ->
             sprintf "%s(%d,%d): %s: %s" (p.Source |> normaliseName) p.Line p.Col d.MessagePrefix d.FormattedMessage
         | Span (s, e) ->
-            sprintf
-                "%s(%d,%d,%d,%d): %s: %s"
-                (s.Source |> normaliseName)
-                s.Line
-                s.Col
-                e.Line
-                e.Col
-                d.MessagePrefix
-                d.FormattedMessage
+            // If both points are on the same line then we can use the a more
+            // compact format.
+            if s.Line = e.Line then
+                sprintf
+                    "%s(%d,%d-%d): %s: %s"
+                    (s.Source |> normaliseName)
+                    s.Line
+                    s.Col
+                    e.Col
+                    d.MessagePrefix
+                    d.FormattedMessage
+            else 
+                sprintf
+                    "%s(%d,%d,%d,%d): %s: %s"
+                    (s.Source |> normaliseName)
+                    s.Line
+                    s.Col
+                    e.Line
+                    e.Col
+                    d.MessagePrefix
+                    d.FormattedMessage
 
     /// Formatted value for the message
-    member private d.FormattedMessage = sprintf "%s %s" d.Kind.Title d.Message
+    member private d.FormattedMessage = sprintf "[%s] %s" d.Kind.Title d.Message
 
     /// Prefix for the message. Used to summarise the diagnostic kind.
     member private d.MessagePrefix =