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diagrams of a tenaille, a tenaille augmented with a straight face (un pan coupé), and a bonnet or priests cap (with two tenailles) outside a ravelin.

Tenaille (archaic Tenalia) is an advanced defensive-work, in front of the main defences of a fortress which takes its name from resemblance, real or imaginary, to the lip of a pair of pincers.[1] It is "from French, literally: tongs, from Late Latin tenācula, pl of tenaculum".[2]

In a letter to John Bradshaw, President of the Council of State in London, Oliver Cromwell writing from Dublin on 16 September 1649 described one such tenaille that played a significant part during the storming of Drogheda.

... There was a Tenalia to flanker the south Wall of the Town, between Duleek Gate and the corner Tower before mentioned;—which our men entered, wherein they found some forty or fifty of the Enemy which they put to the sword. And this 'Tenalia' they held: but it being without the Wall, and the sally-port through the Wall into that Tenalia being choked up with some of the Enemy which were killed in it, it proved of no use for an entrance into the Town that way. ...

—Oliver Cromwell[3]

Tenaille were a development in fortification design formalised by Vauban. To allow the defenders to access the ditches that front a curtain wall a postern was placed low down in the curtain walls close to its centre. To protect the postern Vauban conceived the plan of establishing a work between the flanks and before the curtain, and to which he gave the name of tenaille. This work masks not only the postern, but likewise the flanks and almost the whole curtain. Suitably planned and organized, the tenaille possesses many important advantages, but in whatever manner it is designed, the tenaille will mask the fires of the flanks, and the ditches in front of it will give some cover to attackers.[4]

The first trace that Vauban adopted for the tenaille after inventing it, was to draw it like a small front placed parallel to the curtain, and with its two small flanks parallel to those of the enceinte. But he abandoned this plan, and substituted for it the re-entering angle formed upon the perpendicular by the lines of defence. His tenaille is eloigned 8 to 10 metres (9 to 11 yards) from the flanks and from the curtain opposite the re-entering angle.[4]

By the end of the Napoleonic Wars Vauban's design had been augmented with a straight face (un pan coupé) parallel to the curtain wall (see the middle diagram). The position of its magistral is found by laying off 24 to 25 metres (26⅔ to 28 yards) upon the perpendicular, measuring from the magistral of the curtain; and 10 metres (11 yards) to find its gorge-line. By this plan there is formed in rear of the tenaille a kind of place of arms, of great advantage for debouching into the ditch: the parapet of the tenaille was made 5 metres (16⅔ feet) thick. The profiles parallel to the flanks of the enceinte, could be rounded; and the extremities of the covering line for a length of 3 or 4 metres (10 to 13⅓ feet), could be broken inwards. The tenaille could be crossed along its centre by a great postern leading under its terra-plain; and when the ditch was filled with water, this vaulted passage served as a harbour for boats.[5]

See also