Overcalling, just like opening the bidding, is a way to tell your partner which is your best suit. When you have one long suit, that’s obviously the one you will bid, but there are a few differences that apply when one of your opponents has bid before you:

• You must have at least five cards in your suit. Unlike when you open 1D or 1C openings, where you may have only four diamonds or only two clubs, an overcall promises at least five cards.
• You don’t need as much strength as you do to open the bidding as long as you have room to bid your suit at the one level. A one-level overcall only promises about 9+ HCP, although you might well have quite a bit more than that. If your suit ranks below opener’s, so you need to bid at the two level, you should have the same strength as you need to open the bidding (12+)
• Some hands are too strong to overcall. With 18+ HCP you should always start with a double of the opening bid. Then, when you bid your own suit or NT next, your partner will know you have a very strong hand.
• You can also make pre-emptive overcalls. Just like with pre-emptive opening bids, we show these weaker, distributional hands by going to a higher level than we need to. They are called weak jump overcalls.
• As a general guideline, you should try only to ovecall suits where you have good cards. Asking yourself ‘would I like partner to lead this suit if the opponents end up declaring?’ is a good indicator