The historical Bowie was not a single design, but was a series of knives improved several times by Jim Bowie over the years. The version most commonly known as the historical Bowie knife was rather large and of massive construction, as knives go, usually having a blade of at least six inches (15cm) in length, some reaching 12 inches (30cm) or more, with a relatively broad blade that was an inch and a half to two inches wide (4 to 5 cm) and made of steel usually between 3/16" and 1/4" thick (from 4.8 to 6.4 millimeters). The back of the blade often had a strip of soft metal (normally brass or copper) inlaid intended to catch an opponent's blade, and also often had an upper guard that bent forward at an angle, also intended to catch an opponent's blade. The back edge of the curved clip point, also called the "false edge," was often sharpened in order to allow someone trained in European techniques of saber fencing to execute the maneuver called the "back cut" or "back slash." A brass quillon was attached to protect the hand, usually cast in a mold. It is likely that the blade shape was derived from the Spanish navaja clasp knives carried in Spain and the Spanish colonies in the Americas.