Mohan convention

Mohan is a bridge convention used to intervene over opponents' 1NT opening. Typically it only applies if the opponents are using a weak 1NT opening. Its invention is usually attributed to John Mohan.
Over 1NT:

Double shows 14+ points, and is often left in for penalty. If it is not, the advancer bids as though his partner had opened 1NT..
2 shows both majors, possibly 5-4. If responder does not have a preference, he bids 2 to let opener pick.
2 is a transfer to hearts,
2 is a transfer to spades,
2 shows spades and a minor,
2NT shows hearts and a minor,
3 of a minor shows a long minor and is constructive,
3 of a major is pre-emptive.