That's only the syntax of s-expressions - a data format. The Lisp syntax is defined on top of s-expressions. For example Common Lisp has a CASE operator. That

That's only the syntax of s-expressions - a data format. The Lisp syntax is defined on top of s-expressions. For example Common Lisp has a CASE operator. That's the EBNF syntax: case keyform {normal-clause}* [otherwise-clause] => result* normal-clause::= (keys form*) otherwise-clause::= ({otherwise | t} form*) An example: (case id (10 (foo)) (20 (foo) (bar)) (otherwise (baz))) The expressions are written using s-expressions as data. But still there is structure in those s-expressions, described by the EBNF syntax of CASE. Every special operator and every macro provides syntax. Since users can write macros themselves, everybody can extend the syntax. On top of s-expressions.