Unusual NT

Unusual NT is a convention which overlaps with Michael's. This is usually a preemptive bid  so only use at favourable vulnerability, and have at least two honours or most of your points in the suits you are describing. 

If Opponents have opened at the one level, the 2NT bid is used as an overcall showing at least 5 cards in the two lowest unbid suits. Only use with Weak (6-9 HCP) or Strong (16+ HCP) hands.

1S - 2NT means Clubs & Diamonds

1H - 2NT means Clubs & Diamonds

1D - 2NT means Clubs & Hearts

1C - 2NT means Hearts & Diamonds

Without intervention, your partner (the Advancer) must respond. 

With a weak hand and little support she  chooses from these unbid suits, either her longest suit or, if her suits are of equal length, her cheapest bid.

With 3 or more card support (a fit!) she bids according to her strength taking account of Losing Trick Count. She can reasonably assume the 2NT Overcaller has a seven loser hand.

With a Weak hand the Overcaller passes. With a Strong hand the Overcaller raises the bidding to invite to game. The Advancer then decides the final contract.

However, with a strong hand (16+) and an interest in game or slam, the Advancer would have cue bid the opponent's suit. This bid is forcing.

With stops in the Opponents suit and game points (19+), the Advancer would have bid 3NT to play. Stop there!

Watch out  - there are two exceptions if partner overcalls 2NT:

1. 2NT over a preemptive Opening Bid shows Stops and 18+ HCPs

2. When the Opening Bid is followed by 2 passes a bid of 2NT is Natural showing a balanced hand of 19-22 HCP.