APPEND function concatenates list arguments into one list. Resulting list is shallow copy of specified lists except for the last which is directly shared. See also MAPCAN, CONS, LIST, LIST*.

(append) => NIL
(append '(1 2 3)) => (1 2 3)
(append '(1 2 3) '(4 5 6)) => (1 2 3 4 5 6)
(append '(1 2 3) '(4 5 6) '(7 8 9)) => (1 2 3 4 5 6 7 8 9)
(let ((x '(tail list))) (eq x (cddr (append '(front list) x)))) => T