Glass running pliers are a specialized tool used in stained glass and glass cutting work.
 
Here's what you need to know:
What they do: They apply even pressure along a score line on glass to propagate a clean break. Rather than snapping glass by hand (which risks uneven fractures), the pliers concentrate force precisely on the score, giving you a controlled, clean cut.

How they work: The curved jaws — one convex (bottom) and one concave (top) — create opposing pressure at the center of the score line, causing the glass to run (split) along it.
Key features:
•    Adjustable center guide/mark to align with your score line
•    Rubber or plastic jaw tips to avoid scratching or chipping the glass
•    Usually, 8 inches long for good leverage
 


How to use them:
1.    Score your glass with a glass cutter
2.    Align the center mark of the pliers with the score line at the edge of the glass
3.    Squeeze gently — the break should run along the score

Common uses: Stained glass projects, mosaic cutting, art glass, and general flat glass work.
They're especially useful for straight cuts and gentle curves on thinner glass. For tight curves, you'd typically use grozing pliers or a glass grinder instead.