The generally accepted HCP for one-level overcall is about 8-16. The bottom end reflects whatever the partnership believes represents a reasonable degree of risk and the upper end reflects the maximum number of HCP that does not qualify for a double and new suit bid
An overcall generally shows a five-card or longer suit but an overcall can be made in a very good four-card suit if there is no other reasonable alternative.
Responses to an overcall
~Because the overcall can be made with as few as 8 HCP, a decision is required by the partnership as to what a bid of a new suit by partner means: forcing, non-forcing constructive, or non-forcing.
~Arguments can be made for either type response, and the following recommendation is the most conservative: play a new suit by responder as non-forcing. This is based on the possibility that responder has neither a fit for partner (over-caller) nor a stopper in the enemy suit. A bid of a new suit is made in hope of finding a better fit.
~What if responder has a full opener or better?
~with no fit but a stopper in the enemy suit, a jump in NT may be possible.
~with a fit, a cue-bid of the enemy suit may be make with 11+ points.
~when partner bids a new suit, over-caller with a full opener will normally have a second bid. Without a second bid, the hand is probably a misfit and should be passed out at a low level.
~A meaning must be placed on a jump raise: forcing, invitational, or weak.
~~Recommendation: treat the jump raise as forcing and use a cue-bid to show a limit-raise or better.
~~If a single raise is weak and a cue-bid forcing, the logical definition of a jump raise is invitational/preemptive. The number of trumps held and the vulnerability situation also plays a part in the decision. No definition will succeed at all times, but the following approach is functional:
~With favorable vulnerability, play the jump raise as preemptive and showing either four trumps and four HCP or 3 trumps with a top three honor and at least 6 HCP with distribution values or about 8-10 HCP otherwise. Use the stronger option with unfavorable vulnerability.
Thus, with favorable vulnerability, the call would serve to make the bidding more difficult for the opponent without undue risk. If vulnerable, the bid would be both preemptive and invite partner to game in a major if he overcalled with 15-16 HCP.
~Some hands just cannot be fitted into such clear patterns. When those situations arise, player judgment is important. When overcaller’s or responder’s point range cannot be determined and a decision must be made to let the opponents play the contract or compete, the decision should be passed to the hand with unknown strength. The partner with more strength than has been shown can choose to bid again. Thus, if the overcaller has been supported by partner and has a full opener, he may decide to bid again; but if holding less than a full opener, probably should pass and defer the bidding decision to partner.
~It was once popular to play strong jump overcalls, but the current style is that all jump overcalls are weak. The reason that weak jump overcalls have become standard is that there is another way to handle hands that are too strong for a simple overcall–start with a double and then bid your suit.