Modern, rough-cutting bow saw.
Traditional, woodworking bow saw.
'Bow saw' can mean two totally different types of saw. The kind of bow saw sold in modern hardware stores is a saw with a metal frame shaped like a bow, which has a coarse wide blade. It is used for rough jobs such as for cutting branches off trees.
The other kind of bow saw is a traditional woodworking tool, also known as a turning saw, with a narrow blade (1/4" or less), used for both straight or curved cuts. The blade is stretched under tension between two long narrow handles that are supported and separated by a narrow stretcher in the center of the handles, making an H shape. The blade is kept in tension with a turnbuckle or a twisted cord that is attached to the opposite ends of the handles. If a cord is used, the cord is twisted with a toggle attached to one loop of the cord, adding tension. The toggle hits the center rod, which keeps the cord from untwisting. held in tension. Both the handle and knob (at the other end of the blade) can be turned so that a cut can be made more or less parallel to the required cut.
The turning saw version of the bow saw is available with blade lengths of 25 to 40 centimeters (10 to 16 inches). Generally both hands grips the one handle, hence the bulbous shape with narrow neck, but when thick wood has to be sawn it is helpful to have a person at each side, both sides of the wood having been marked. In this way it is much easier to keep the cut square to the sides of the timber. The rivets holding the blade to the handle can be withdrawn so that the saw can be used for an internal cut, the blade being threaded through a hole drilled through the timber.