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