It sometimes has also a demi-lune or bonnet, as in the case of demi-tenaillons.

(Fig.

43.) A _crown-work_ consists of two fronts of fortification, and two wings.
(Fig.

44.) It is sometimes made _double_, and even _triple_.
These works are also employed as advanced works, and placed entirely in front of the glacis.

They have generally been added to a fortress for the purpose of occupying some important piece of ground not included within the limits of the main work.