See chapter 8 of the USBC Playing Rules
6. Holes.
a. Holes or indentations for gripping purposes shall not exceed five and shall be limited to one for
each finger and one for the thumb, all for the same hand. The player is not required to use all finger holes in any speci c deliver y, but they must be able to demonstrate, with the same hand, that each gripping hole can be simultaneously used for gripping purposes. Any thumb hole that is not used for gripping purposes during the delivery would be classi ed as a balance hole.
b. One hole for balance purposes, not to exceed 11⁄4 inch in diameter is permitted. (Slugs and bowling tape will be allowed in balance holes, however interchangeable devices are
not allowed).
1) Any hole that cannot be reasonably shown to be used with a single hand is classi ed as a balance hole.
2) Balance holes may not intersect gripping holes.
c. One vent hole for each nger and/or thumb hole, not to exceed 1⁄4 inch in diameter is permitted.
d. One mill hole for inspection purposes not to exceed 5 ⁄8 inch in diameter and 1⁄8 inch in depth
is permitted.