When planning the matting of a picture, many artists use a pair of “L”s cut from mat board to crop the picture and select the best opening size to cut in the mat. In the process, two questions come to mind: Is my test opening a true rectangle and what size opening do I have? Adding scales (rulers) on the “L”s provides a quick and easy answer to these questions without having to fumble with an additional ruler. The opening size is read where the inner edges of the “L” intersect the corresponding scale.
Make the paper scales by copying a ruler with clear divisions and numbers on a good quality copy machine. Eighteen-inch scales can be copied on ledger paper by placing the ruler across the diagonal of the copy area. Four scales are best, one for each leg of each “L.”
Cut off surplus paper so you have the numbers and divisions in a strip about ? to ? inches wide, and a little paper remaining below the “0” of the scales. Carefully extend the inner edges of the “L”s with pencil lines as shown in the inset, and locate the zero-point of the scales on those lines. Glue the scales to the outer edge of the “L”s so the divisions are as close to the edge as possible. Use rubber cement for the gluing to ensure the scales stay in place and they’re not stretched or distorted.