OmniGrip Non-Slip 3.5 Square Ruler. This little ruler includes a Â½" seam allowance, making it the perfect size template for 3" finished squares. Useful for cutting and trimming miniatures and checking seam allowances. 3.5" Square is used for traditional blocks and has diagonal lines for triangle cuts. Omnigrid brings easy to see, even easier to cut rulers to you with the Omnigrip ruler. Special non-slip backing eliminates slipping when measuring and rotary cutting. Neon green double-sight lines that seem to glow on fabric make measuring accurately a breeze.