What is the difference between a you and a t shank jigsaw blade?

The U-shank has a U-shaped cut at the securing end of the blade, while the T-shank has a pointed end with 2 knobs sticking out on either side.

U-shank blades used to be the norm when the blades were manually fastened in the jigsaw machine, but they are now gradually being replaced by T-shank blades that were instroduced for easy fastening.

Many popular jigsaw machines will accept both types, and typically both can be fastened with an easy single-click mechanism now. If your machine accepts either, there is no functional difference.