BT Flange Taper(BonsainTaper): BT-Flange was developed in Japan and is also a dual flange taper. Tooling comes in a range of sizes designated as BT 30, BT 40, BT 50, etc. and uses the same NMTB body taper as CAT 40. BT tooling is symmetrical about the spindle axis, which CAT tooling is not. This gives BT tooling greater stability and balance at high speeds. BT Tool Holders will accept both Imperial and metric sized tools. BT Tooling looks very similar and can easily be confused with CAT tooling. The difference between CAT and BT is the flange style, thickness, and the thread for the pull stud is a different size. BT Tool Holders use Metric thread pull studs (retention knobs).