TENAILLON, or Great Tenaille, in Fortification, is a species of exterior work which has been occasionally constructed before the faces of a small ravelin, with a view of increasing the strength of the latter, procuring additional apace beyond the ditch, or covering the shoulders of the bastions. They were invented by Vauban, who, however, very seldom constructed them; and subsequent engineers have generally considered them as inferior in defensive qualities to a counterguard [Cl Q. FORTIFICATION, fig., cola 171-172,] placed over the faces and salient angle of the ravelin.
The form and position of a tenaillon may be understood, r being supposed to represent a small ravelin, if beyond the ditch of the latter the ramparts of the right and left faces be produced till each of them meets a rampart nearly perpendicular to the face of the bastion and extending to the place of meeting from the counterscarp of the main ditch at a point opposite the middle of that face. The works thus formed, one over each face of the myelin v, constitute a tenaillon ; before each line of rampart is a ditch, and part of the general covered way, the main ditch and that of the ravelin being in the rear. The two faces which are beyond the salient angle of the ravelin would, if produced towards the latter, form with each other a re-entering angle, whose vertex would coincide with that of the said angle.
The objections to tenaillons are, that the besieger would experience little difficulty in establishing a lodgment on that part of the covered way or glacis which is immediately in front of the salient angle of the ravelin; and in this situation he would be able to breach the faces of the two ballbaations in four places, by fires of artillery directed along the ditches of the ravelin and those on the aide faces of the tenaillon.
The salient angles of the tenaillon, and of the ravelin which it covers, may be breached at the same time, and, when the ditches are dry, it would be possible to attack and carry the raveliu at the time of making the assaults on the tenaillon : then, the enemy having got possession of the former work, any retrenchments which may have been made in the tenaillon must necessarily be abandoned by the defenders.
The re-entering space between the two faces which are in the pro longation of the faces of the ravelin, and which constitute the head of the tenaillon, is sometimes occupied by a small redout, consisting of two ramparts perpendicular to the faces which have been just men tioned; and thus there may be obtained a good crossing fire for the defence of that part of the covered-way which is concealed by the salient angles of the tenaillons from the defenders of the bastions.
Demi-tenaillons are works placed also on the aides of a ravelin, and consisting of two ramparts which are perpendicular to and nearly opposite the middle of the faces of the bastions and ravelins : these are usually accompanied by counterguards which cover the salient angles of the latter works, and are called Bonnets.