Overcalls

One level simple overcalls: 8-16
Two level simple overcalls: 10-18

Overcalls should depend on length, vulnerability, and suit quality.
Better hands are required for vulnerable overcalls.

My personal preference is that an overcall be based on at least five cards, but often bidding four card suits (with a good suit and at least an opening hand) can be worthwhile.
With 13-17 HCP and a 4 card suit of at least KJTx (three of top five OR two top honours), I'll usually overcall [and will have four of opener's suit and a singleton in another suit (usually 4-4-4-1 shape)].

For example with KQT4 AJ98 KT83 8, if RHO opens 1, I'll overcall 1.
Best after that is usually to be content with playing a partial unless partner gives a limit raise.
Doubling them later for penalties is often their unpleasant surprise.

Mike Lawrence's book Overcalls, even with the premise of "negative free bids", is instructive with many examples.

Basically, as a responder to partner's overcall, I will have
  • WITH A FIT
  • 6-10 for a single raise
  • 11-13 for a limit raise (cuebid)
  • 14+ (cuebid, followed by game)
  • 4+ card support for pre-emptive jumps
  • NO FIT
  • 10+ for a new suit (FORCING and denying support for partner)
  • 9-11 to bid 1NT and a stopper in opener's suit;
  • 12-14 for 2NT
  • 15+ for 3NT
  • -none of my no-fit bids promises ANY cards in partner's suit
    -all of those overcall responses DENY a good five card major


    My Jump overcalls are weak (better vulnerable)

    Also important are RESPONSIVE DOUBLES! Often I have points and both the other suits.

    Practice your overcalling and responses with partner

    AFTER AGREEING ON HOW YOU PLAY AND RESPOND TO OVERCALLS, follow a link with your partner to some hands where one of you holds the South hand and (maybe) overcalls East's opening bid of 1 (or 1 if your suit is clubs).

    Write down your responses.

    Then switch directions and bid them again, again writing down your responses.

    Compare and argue (discuss). That's half the fun and there's noone around to hear you rant.
    Overcaller
    Club
    Sets
    Diamond
    Sets
    Heart
    Sets
    Spade
    Sets
    Responder
    Club
    Sets
    Diamond
    Sets
    Heart
    Sets
    Spade
    Sets
    One Hand Overcalling
    Club
    Sets
    Diamond
    Sets
    Heart
    Sets
    Spade
    Sets