My B992 which should be the same as a B901 measures 3 1/8 - 3 1/4 inches or about 78-83mm center-to-center. I have relatively small hands and this span is very manageable for me as is the rest of the keywork on the Yani bari. The question made me curious so I also measured my Barone bari as well as my Yamaha and Barone tenors. The Barone bari and Yamaha were virtually identical to the Yani bari while the Barone tenor was just a little wider than the other 3 horns at 3 1/4 - 3 3/8 inches.