[Ayoka]

The AKC rules seem to suggest it is up to the judge to determine what is acceptable, so long as it is not a prong or an electric collar.

In Canada, you can't have a prong in obedience in the ring...but you can have a slip (choke) martingale or plane buckle or quick release - head halters are also not acceptable. Same for just about every venue we have been in, CKC & AAC Agility & Flyball (where you can't have an e-collar or prong on the grounds at all and head halters are permitted but not in the ring)