BillPatch, on 2015-August-28, 21:15, said:
In some systems the offensive bidders have guaranteed a club fit; in many others it's implied. Thus, despite the unfavorable vulnerability, this is a prebalancing situation, so the two spade overcall does not show a huge hand. Responder is limited and opener has not promised extra values, so overcaller should have at least constructive values. Thus I expect about an opening hand with a five card suit and 7LTC as a minimum, and also many hands with 6 LTC. Vulnerable at IMPs I want to be in game opposite the 6 LTC hands. While I have only two proven cover cards, an extra trick may be developed by ruffing a second loser in clubs, or dropping a trump queen, or some combination using the JTxx plus value in diamonds. I will make a weak game try by advancing 3♠.
While making the game try will increase the possibility of going down in a part score. 3♠ preempts 3♣ by the opps, either as a contract or encouragement for 4♣. 3♠ is relatively safe since we have a 9+ ♠ fit.
In yet other systems the offensive responder denies as many as 4 clubs. All the authorities presenting in Standard American with four card majors recommend the 2 club raise here with a minimum hand with 4 clubs. With 5 card major systems, many authorities prefer to use inverted minor raises. in which a weak hand with club support requires 5 to make a raise(to 3) more balanced raises used the 1NT response.
If their methods deny the 4 card raise, they frequently will not have an eight card fit, so this is not a pre-balancing situation, so the two spade overcall should show substantial extra values, probably one LTC better than the pre-balancing hand. Since hands with 4 diamonds may bid one diamond rather than 1 NT, if they play inverted minors, 4 card support is implied by the NT bid, so against the normal ACBL player, who uses this common convention--pre-balance. If their convention card fails to specify--ask!