elisp: Dynamic Binding
11.9.1 Dynamic Binding
----------------------
By default, the local variable bindings made by Emacs are dynamic
bindings. When a variable is dynamically bound, its current binding at
any point in the execution of the Lisp program is simply the most
recently-created dynamic local binding for that symbol, or the global
binding if there is no such local binding.
Dynamic bindings have dynamic scope and extent, as shown by the
following example:
(defvar x -99) ; ‘x’ receives an initial value of −99.
(defun getx ()
x) ; ‘x’ is used free in this function.
(let ((x 1)) ; ‘x’ is dynamically bound.
(getx))
⇒ 1
;; After the ‘let’ form finishes, ‘x’ reverts to its
;; previous value, which is −99.
(getx)
⇒ -99
The function ‘getx’ refers to ‘x’. This is a “free” reference, in the
sense that there is no binding for ‘x’ within that ‘defun’ construct
itself. When we call ‘getx’ from within a ‘let’ form in which ‘x’ is
(dynamically) bound, it retrieves the local value (i.e., 1). But when
we call ‘getx’ outside the ‘let’ form, it retrieves the global value
(i.e., −99).
Here is another example, which illustrates setting a dynamically
bound variable using ‘setq’:
(defvar x -99) ; ‘x’ receives an initial value of −99.
(defun addx ()
(setq x (1+ x))) ; Add 1 to ‘x’ and return its new value.
(let ((x 1))
(addx)
(addx))
⇒ 3 ; The two ‘addx’ calls add to ‘x’ twice.
;; After the ‘let’ form finishes, ‘x’ reverts to its
;; previous value, which is −99.
(addx)
⇒ -98
Dynamic binding is implemented in Emacs Lisp in a simple way. Each
symbol has a value cell, which specifies its current dynamic value (or
absence of value). 
Symbol Components. When a symbol is given a
dynamic local binding, Emacs records the contents of the value cell (or
absence thereof) in a stack, and stores the new local value in the value
cell. When the binding construct finishes executing, Emacs pops the old
value off the stack, and puts it in the value cell.
Info Catalog
elisp: Variable Scoping
elisp: Dynamic Binding Tips