Perfect for engineering and construction applications. Every Mayline parallel ruling straightedge begins with a durable black phenolic laminate base where a transparent blade is inset for see-through registration and fine line quality. For those who work with a variety of surfaces straightedge is designed with retractile ball-bearing rollers of metal or Delrin built right into its base. They literally float over any surface whether it’s paper or film. The rollers come in chrome-plated brass or plastic. Plastic rollers are recommended for use with drafting films. Above-board mounting hardware included.