The bowstring should be composed of several dozen turns of thin twine, of pure hemp or flax. What is known in seaports as 'sailmakers sewing twine' is excellent for the strings of large crossbows, as it is very strong and will not stretch under the great strain of the steel bow. Any twine in the form of soft twisted string is sure to stretch, and what was at first a taut bow-string will - if this kind of material is used - soon become a slack and useless one.

For small crossbows, such as those presently described for target practice and rook shooting, there is no twine so suitable for their bow strings as that employed by bookbinders for stitching the leaves of a book to its back.

Bookbinder's thread is extremely strong and hard, and though it will not stretch, it is rather too fine for the string of a large crossbow.

The string of a crossbow should always be taut, so that when the weapon is discharged, the bolt receives sufficient impetus at the moment the bowstring is checked by the ends of the bow, as the latter straightens.

In some foreign longbows, an arrow can be driven as far with a loose bow-string as with the more usual tight one, but in a crossbow the draw of the string along the stock is so short (from 5 in. to 6 in. only), that all the power of the bow has to be utilised in this small length of pull.

In mediaeval pictures, the string of the crossbow is often represented with the same thickness at its looped ends as at its centre. This suggests that in old days, the string was taken its full thickness round each extremity of the bow, and that the ends of the string were then in some way knotted or wrapped, to secure them and form the necessary loops.