let

(let ((var init)...) body...)

Establishes local variable bindings: each init is evaluated (in the surrounding scope, so the bindings are parallel, not sequential) and bound to its var for the duration of body. The body forms are evaluated in order and the value of the last one is returned; with an empty body the result is nil. The bindings are variable bindings only -- per Lisp-2 they do not shadow the function namespace, so a let-bound car does not affect calls to the function car.

If a var names a variable that was proclaimed special (by defvar/defparameter or (declaim (special ...))), or a (declare (special var)) at the head of body names it, that binding is dynamic: it is visible to any function called during body, not just lexically nested code, and is restored when body exits. A reference to a special variable reads the binding in effect when it runs, in a closure as anywhere else, so a closure built in body and called after the let has exited reads the global value. Every init runs before any variable is bound, so an init that calls a function reading a special sees the outer binding, not the one this let establishes. Ordinary (non-special) names are bound lexically as usual, and so is a name that only a local declaration elsewhere makes special: such a declaration covers its own binding and the references in its body (declare). See also progv for a runtime-computed list of special bindings.