When you want to draw a line that is square with your drawing paper, or use drafting triangles on, a parallel straightedge bar will perform the job with ease. Parallel straightedge bars are great for making quick sketches before reproducing your ideas in cad. They also are great tools for drawing guidelines when lettering your drawings. Many Design Professionals still use parallel straightedges even though the world of manual drafting has almost been erased by computers. These products are also called Mayline Parallel Straightedges, Parallel Drafting Bars, Parallel Bars, and Parallel Straightedges.
Parallel straightedge replacement parts can be found here.
Mayline Parallel Straightedge Options Explained
E-Z Grip Cap — The E-Z grip cap is a handy 5/16" raised gripping bar that allows for easy maneuverability and a storage place for lead holders or erasers.
Armoredge — Armoredge is a nearly invisible protective strip of stainless steel along the edge that protects against nicks and dents from drawing tools.
Delrin Rollers — Delrin plastic rollers are best for Mylar.
Metal Rollers — Metal rollers are best for vellum and other paper media.