If it is false, the pretty printer is not used and a minimum of whitespace₁ is output when printing an expression.

If it is true, the pretty printer is used, and the Lisp printer will endeavor to insert extra whitespace₁ where appropriate to make expressions more readable.

*print-pretty* has an effect even when the value of *print-escape* is false.

 (setq *print-pretty* 'nil)   NIL
 (progn (write '(let ((a 1) (b 2) (c 3)) (+ a b c))) nil)
(LET ((A 1) (B 2) (C 3)) (+ A B C))
  NIL
 (let ((*print-pretty* t))
   (progn (write '(let ((a 1) (b 2) (c 3)) (+ a b c))) nil))
(LET ((A 1)
      (B 2)
      (C 3))
  (+ A B C))
  NIL
;; Note that the first two expressions printed by this next form
;; differ from the second two only in whether escape characters are printed.
;; In all four cases, extra whitespace is inserted by the pretty printer.
 (flet ((test (x)
          (let ((*print-pretty* t))
            (print x)
            (format t "~%~S " x)
            (terpri) (princ x) (princ " ")
            (format t "~%~A " x))))
  (test '#'(lambda () (list "a" #\b 'c #'d))))
#'(LAMBDA ()
    (LIST "a" #\b 'C #'D))
#'(LAMBDA ()
    (LIST "a" #\b 'C #'D))
#'(LAMBDA ()
    (LIST a b 'C #'D)) 
#'(LAMBDA ()
    (LIST a b 'C #'D))
  NIL