the work of a backsaw requires a thin, stiff blade. these two immediately incompatible requirements are resolved by swaging (and perhaps, spot welding) a stiffening cap over the length of the top edge of the blade. this thickening of the blade is the back and is generally made from some kind of metal, usually brass or steel. the stiffening back is carried through into the handle so that the blade does not bend between the two. the thickening of the back limits the depth to which the saw can cut but this is generally not a limitation in their typical uses.