Serious 3NT

Serious 3NT is a contract bridge bidding convention. It is used in game-forcing auctions where a major suit has been agreed upon.
1–2 if using 2/1 Game Forcing or 2–3
The next bid indicates suitability for slam:

3NT: strong
4/4/4: cuebid, but not strong.
4: truly minimum.

Since Spades are the agreed trump suit, Three No Trump is not usually the best contract. Using Serious Three No Trump, a bid of Three No Trump says "I am
Serious about slam", i.e. "I have a good hand". Bypassing bidding Three No Trump says "I am not Serious about Slam". Examples shown in the above auction.
If one player bypasses 3NT with a Cuebid, then a Cuebid by partner indicates that he is serious. Note that in the example auction above, if the strong hand is next, he can differentiate between a strong hand, a weak hand that has a control or a minimum hand.
Serious Three No Trump was devised by and named by Eric Rodwell.

Frivolous 3NT

Some partnerships reverse the meaning of 3NT in this situation. A weak hand bids 3NT to indicate no additional strength. This has the effect that a weak hand will not pinpoint a control for the defense.