Although I ordered the 1/4" shank version, the first set of bits I received were 1/2" shanked and the box was clearly with Marked 1/2" which did not fit my 1/4" router chuck so I sent them back for an exchange. The second set, as you might have guessed, also had a 1/2" shank, as stated on the label. This is Revain's fault as the product was correctly labeled by the retailer. In the meantime, when using others have been identified Bits snapped the frames of my old router which is over 40 years old so it was over. I bought a new one with spare 1/4" and 1/2" chucks and used it with my new Bosch router. Router, Bosch router table (also new) and these KOWOOD 1/2" shank and groove bits work GREAT! I have cut eight shaker style doors so far and they fit perfectly. I added a shim (included) to make the groove a bit wider and space the outside knives apart, and they match the actual 1 x 3.3/4" thickness perfectly.