Beam for Steel Beam Trammels, 20"/600mm Length. Starrett C251 Series Stell Beam Trammels and Attachments are rigid, well-designed trammels for layout, scribing, and measuring distances and circles. Each trammel features flattened top beam for claming and spring-friction to prevent sliding, and a knurled swivel grip for easier swinging when scribing circles. This model provides a maximum dividing range of 18", maximum circle scribing diameter range of 36" and beam size of 20". Coupling with extra 20" (600mm) Beam 20".