Cappelletti

Also known as Hamilton. This is a defensive bid after a 1NT opening. The meanings to the overcalls are as follows:

Double Strong hand. Intended of penalties
2C Shows a one suited hand. Partner should respond 2D, but can pass with a good 6-card club suit. 2C bidder then passes with diamonds, or corrects to their suit
2D Staymanesque bid showing 5/4 in the majors. Partner can pass with little in the majors but good long diamonds, bid 3C with good long clubs, or bid a major. Bidding 3 of a major is preemptive
2H Shows hearts and a minor. Partner can pass with heart support, or bid 3h as a preemptive measure. To discover the minor suit, partner bids 2NT
2S Shows spades and a minor. Responses are the same as for 2H
2NT Shows both minors. Partner either bids their preferred minor, bids a major with 11+ points and a good suit, passes or bids 3NT with 11+ points and good major stops.

The use of Cappelleti depends on your bidding style:

Conservative

2C 13-15pts, 6 Cards
2D 13-15pts, 5-5 in the majors
2H, 2S 13-15pts, 5 Cards

Aggressive

2C Equivalent to a Weak Two Bid
2D 9+pts, 4-4 in the majors
2H, 2S 9+pts, 4 card major, 5 cards minor
Double Strong hands should be opened with a Takeout Double