4.5" Half Square Triangle Ruler from Bloc-Loc. 

• Use for squares up to 4.5" (can be used for squares smaller than 4.5")
• Patented Bloc Loc channel is machined down the middle
• Locks seam allowance in place for ease in squaring up half square triangle blocks
• No turntable necessary, just turn the ruler & your fabric moves with Bloc Loc
• Solid lines mark every 1/2" (grid in center portion of ruler)
• Outer perimeter has 1/8" ruler markings
• 1/8" clear acrylic