The Parts of a let Expression ​

A let expression is a list of three parts. The first part is the symbol let. The second part is a list, called a varlist, each element of which is either a symbol by itself or a two-element list, the first element of which is a symbol. The third part of the let expression is the body of the let. The body usually consists of one or more lists.

A template for a let expression looks like this:

emacs-lisp
(let varlist body...)

The symbols in the varlist are the variables that are given initial values by the let special form. Symbols by themselves are given the initial value of nil; and each symbol that is the first element of a two-element list is bound to the value that is returned when the Lisp interpreter evaluates the second element.

Thus, a varlist might look like this: (thread (needles 3)). In this case, in a let expression, Emacs binds the symbol thread to an initial value of nil, and binds the symbol needles to an initial value of 3.

When you write a let expression, what you do is put the appropriate expressions in the slots of the let expression template.

If the varlist is composed of two-element lists, as is often the case, the template for the let expression looks like this:

emacs-lisp
(let ((variable value)
      (variable value)
      ...)
  body...)