A @P04873@ in which vertically drawn bonds are considered to lie below the projection plane and horizontal bonds to lie above that plane. Thus for the molecule Cabcd:
Blue Book, p. 484 [Terms] [Book]
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2208 [Terms] [Paper]
See also:
White Book, 2nd ed., p. 128 [Terms] [Book]