Rules are flat, graduated scales used for length measurement.  Rules and scales are available in variety of forms such as rigid or flexible rules, electronic linear scales, folding rules, drill point gages, tape measures and rule depth gages.  Rules generally provide an approximate measurement compared to precision gages.  However, digital or electronic linear scales designed for OEM machine tool applications and provide a high degree of accuracy.

Rules and scales fall into the general category of measuring tools.  Measuring tools are instruments and fixed gages that provide comparative and quantitative measurements of a product's or component's dimensional, form and orientation attributes such as length, thickness, level, plumbness and squareness.  Measuring or shop tools include rules, linear scales, recticles protractors and angle gages, level and inclinometers, squares and fixed gages.  Measuring tools are used in construction and building (contractors), drafting and drawing (designers), machine shops and tool rooms (machinists), field work (surveyors) and offices.

Reticles are optical scales used in conjunction with microscopes or magnifiers to measure fine details.  Measuring tapes are used measurements of large structures or features in buildings, plants and field work.   Specialized rules or scales are available such as short rules for length measurement in a cavity or shrink rules for pattern making in foundries.