Rule (Ruler)

A rule (ruler) is a straight, graduated measuring tool -- rigid steel rules, folding wood or brass rules, and architect's or engineer's rules (which use specialized, often triangular cross-sections with multiple scaled-down measurement systems for reading a scaled drawing directly) are all variations on the same basic idea. Compared to a flexible tape measure, a rigid rule holds a genuinely straight reference line under its own stiffness, which matters for precision layout work where a tape's slight sag or curve over a distance can introduce real error.

A 6" or 12" steel rule specifically is a bench staple for exactly that reason -- small, precise, straight-edged measurements where a tape measure's larger hook and slight give aren't accurate enough.