These layouts allow bowlers to grip the balls in different configurations, which create different ball reactions depending on which holes the bowler uses to deliver the ball. This has led to questions concerning USBC’s gripping-hole specification. USBC’s specification states:
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 specific delivery, 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 classified as a balance hole.
Also, a ball may have one hole for balance purposes, not to exceed 1¼ inches in diameter at any point through the depth of the hole. (Slugs and tape will be allowed; however, interchangeable devices are not allowed).
The basic USBC interpretation of this specification is that bowlers are allowed one hole for each finger, not multiple holes for each finger.
Layouts such as the duo-grip are considered to have two gripping holes and a balance hole in each orientation. Balance holes are not allowed to have an insert or interchangeable device, and they cannot have grooves, slots, etc., to hold an insert or interchangeable device as that is not the intention of a balance hole. So, basically, for a duo-grip to be legal, inserts or interchangeable devices are not allowed.
Additionally, since the third hole in the duo-grip is considered the balance hole, there cannot be an additional balance hole when this grip is used.
Bowlers still can drill their bowling balls with up to five gripping holes, assuming each hole is intended for a different finger. Tournament directors reserve the right to make the final decision as to the intent being, either, a duo-grip hole or a regular gripping hole which is intended for a different finger.
A few examples to illustrate what would be legal and illegal:
Wi Grip (legal)
The middle two fingers move between all three holes.
Wi Grip (illegal)
This is illegal for two reasons: balance hole cannot have inserts and it has two balance holes.
Triangle grip (legal)
For no-thumb bowlers.
Triangle grip (illegal)
This is illegal because balance hole cannot have inserts.