An L-square ruler is used by drawing a line from the ruler’s corner to the desired number on one of the ruler’s scales. The ruler includes scales for halves, thirds and eighths.
An L-square (or tailor’s square) is a two-armed ruler used to make reduced scale patterns in pattern drafting. It eliminates the need for math when figuring scale measurements. The short arm has halves, 4ths, 8ths and 16ths. The long arm has two-thirds, 3rds, 6ths and 12ths. The tailor first determines which scale to use, then draws a line from the zero point at the corner of the ruler to the number on the desired scale corresponding to the original full-scale measurement. For example, if the full scale pattern calls for a measurement of 16 inches and the tailor wants to make a half scale pattern, the tailor draws to the 16 mark on the half scale, which creates an 8-inch line.