These are used by drywall installers to guide a utility knife in scoring drywall sheets prior to snapping them. They are also used to mark or score a parallel line to one edge. But they are useful in any shop when a relatively long (≈48") straight edge is needed. Usually made of aluminum (sometimes with a plastic resin cross tee), they are about 1⁄8" thick.
Appearances: Episodes 701, 802, 806, 1201, 1206
