by two dark strips, called purfling, runs around the edge of the top and is said to give some resistance to cracks originating at the edge. It is also claimed to allow the top to flex more independently of the rib structure. Some violins have two lines of purfling or knot-work type ornaments inlaid in the back. Painted-on faux purfling on the top is usually a sign of an inferior violin. A slab-sawn bass bar fitted inside the top, running lengthwise under the bass foot of the bridge, gives added mass and rigidity to the top plate. Some cheaper mass-produced violins have an integral bass bar carved from the same piece as the top. Ideally the top is glued to the ribs and linings with slightly diluted hide glue to allow future removal with minimal damage. Back and ribs The back and ribs are typically made of maple, most often with a matching striped figure, called "flame." Backs may be one-piece slab-cut or quarter-sawn or bookmatched two-piece quarter-sawn. Backs are also purfled, but in their case the purfling is less structurally important than for the top. Some fine old violins have scribed or painted rather than inlaid purfling on the back. The small semicircular extension of the back known as the "button" provides extra gluing surface for the crucial neck joint and is neglected when measuring the length of the back. Occasionally a half-circle of ebony surrounds the button, either to restore material lost in resetting the neck of an old instrument, or to imitate that effect. The ribs, having been bent to shape by heat, have their curved shape somewhat reinforced by lining strips of other wood at the top and bottom edges. The linings also provide additional gluing surface for the seams between the plates (top and bottom) and the rib edges. Neck The neck is usually maple with a flamed figure compatible with that of the ribs and back. It carries the fingerboard, typically made of ebony, but often some other wood stained or painted black. Ebony is considered the preferred material because of its hardness, appearance, and superior resistance to wear. Some very old violins were made with maple fingerboards carrying a veneer of ebony. At the peg end of the fingerboard sits a small ebony or ivory nut, infrequently called the upper saddle, with grooves to position the strings as they lead into the pegbox.