diff --git a/doc/language.tex b/doc/language.tex
index 08d2420..e9f2d75 100644
--- a/doc/language.tex
+++ b/doc/language.tex
@@ -10,10 +10,25 @@ Each procedure can get some input and produce an output as a result of
 executing a \textbf{statement block}, a list, where each \textbf{statement}
 is executed in the order it appears in the block.
 
-Statement components are other statement blocks and \textbf{expressions},
-where a statement has control over the evaluation of its components.
-Statements can also modify the state of the procedure or the program by
-mutating variables.
+\chapter{Vocabulary}
+
+A language is an infinite set of sentences, namely the sentences well formed
+according to its syntax. In Elna, these sentences are called compilation units.
+Each unit is a finite sequence of \textit{tokens} from a finite vocabulary.
+The vocabulary of Elna consists of identifiers, key words, numbers, characters,
+strings, operators, delimiters, and comments. They are called \textit{tokens}
+and are composed of sequences of characters.
+
+The following lexical rules must be observed when composing tokens. Blanks and
+line breaks must not occur within tokens (except in comments and strings). They
+are ignored unless they are essential to separate two consecutive tokens.
+Capital and lower-case letters are considered as being distinct.
+
+\section{Comments}
+
+\textit{Comments} may be inserted between any two tokens in a program. They
+are arbitrary character sequences opened by the bracket \verb|(*| and closed
+by \verb|*)|. Comments do not affect the meaning of a program.
 
 \chapter{Expressions}