Veritas Dovetail saw guide.

From the website, "The Veritas dovetail guides, which clamp in position, hold the saw at the correct place and the correct angle for a perfect cut. 3/4" rare-earth magnets embedded in the anodized aluminum guide and covered with a layer of low-friction UHMW plastic keep the saw perfectly aligned as you cut."

For best results use a dovetail saw (we have two!)