In de hoofdgracht gelegen 'buitenwerken' van vestingen, dienende tot dekking van 'saillanten' van 'bastions' of 'ravelijnen'.Triangular outworks situated in the moats of fortifications, and that resemble bastions with a crescent-shaped gorge, constructed to protect a bastion or curtain. The term is often used interchangeably with 'ravelins'. Historically there may have been distinguishing characteristics between ravelins and demi-lunes in the Italian school of fortification design, but in the modern era these terms are most often used synonymously.