A tenail is a fortification outwork consisting of two parallel sides with front, in which is a re-entering angle. It was used from the 16th to 19th centuries.

single tenail
In Graveney's World Even Something as Trivial as an Ingrowing Tenail Couldupset the Delicate Balance
Farm Riparian Land Use and Management: Driving Factors and Tensions Between Technical and Ecological Functions
Proceedings of the Third International Congress on Construction History, Cottbus, May 2009
Cetatea bastionar? de la Chilia
Hvězdárna a planetárium Brno. ?ešení technologické etapy hrubá spodní stavba.
The Indian Railway King
Yvonne Girard, femme, catholique et militante, 1905-1932.
L'angoisse de séparation comme support au trouble du sommeil ou de l'endormissement
L'Homme aux yeux gris