Highly unusual methods (HUMs) are bidding systems which turns the whole bidding mechanism over. Special defences are needed to deal with this kind of systems. They are classified as yellow sticker by WBF, and are forbidden in most of the events, except the highest level of events, in which they are subjected to a high level of restrictions.
The definition of an HUM is a bidding system which exhibit one of the following characteristics:
- A Pass in the opening position shows at least the values generally accepted for an opening bid of one, even if there are alternative weak possibilities. (Strong pass systems)
- By partnership agreement an opening bid at the one level may be weaker than pass. (Strong pass systems)
- By partnership agreement an opening bid at the one level may be made with values a king or more below average strength. (Very light opening bids)
- By partnership agreement an opening bid at the one level shows either length or shortage in a specified suit (Either-or method)
- By partnership agreement an opening bid at the one level shows either length in one specified suit or length in another. (Either-or method)
With the exception to the above being an opening of one of a minor in a strong club or strong diamond system.