Abstract: An easily attachable or removable snap-on adapter to triangular scales provides supplemental length-measuring scales, besides serving as a guard against accidental use of an inappropriate scale. It also acts as an aid for correctly orienting and accurately and firmly positioning the ruler for measurement or drafting purposes. It is made preferably of a molded plastic piece, whose inner contour comprises two elongated beads fitting snugly into each of two grooves along two sides of a standard triangular scale, and whose contour comprises a flap beneath each of these beads and a substantially flat, but flexible portion above the beads. When in use, the part of the inner contour which is directly opposite this flat portion is designed to rest on the upper edge of the scale.