The great and increasing demand for irregular forms to be made from sheet metal has made Triangulation an important factor in Sheet Metal Pattern Development. 'this has induced the writer to lay before the sheet metal worker, a work designed for the purpose of enabling him to acquire a thorough understanding of this branch of Pattern Cutting. Triangulation has in many instances been a subject of more or less mystery. However, from a study of Geo metrical works, we conclude that its secrets have long been known.