The Fourth suit Forcing principle applies when either player bids the as yet unbid 4th suit at the 2-level or as a regular non-jump bid at the 3-level.
A bid of the 4th suit shows weakness in that suit.
(With a genuine 5-5+ suited hand, bid the 1st suit at the 1-level, then bid the 2nd suit as a jump-shift usually at the 3-level.)

The RESPONDER can make a 4th suit forcing bid with 11+ points (or 10 pts and 8 losers).
	Responder's hand :	♠ J 10 3   ♥ K Q 10 8 5   ♦ 7 6 2   ♣ A J 6 5
	Bidding :	1♣ - 1♥ - 1♠ → 2♦
Opener's reply to Responder's 4th suit forcing bid, in order of their priorities, are : Show delayed 3-card support for Partner's (likely) 5-card ♥ or ♠ suit. Bid NT with a stopper in the 4th suit Show delayed 3-card support for Responder's (likely) 5-card ♣ or ♦ suit. Raise the 4th suit. If Opener could genuinely hold the 4th suit, his raise shows a 4 card holding in the 4th suit. If Opener earlier denied holding the 4th suit, he is showing a strong hand (16+ points) but with no stopper in the 4th suit. Failing any of the above, Opener rebids his own 5-card suit Opener must bid again if 4th suit forcing Responder makes another bid ! The 4th Suit bidder may Pass at any time.
The OPENER can bid a 4th suit forcing bid with a weak Opening hand or better.
	Opener's hand :	♠ K 3   ♥ Q 10 7   ♦ A K 10 8 7 6 2   ♣ 5
	Bidding :	1♦ - 1♠ - 2♦ → 2♥ → 3♣
Responder's reply to Opener's 4th suit forcing bid, in order of their priorities, are : Show delayed 2-card support for Opener's 5+card ♥ or ♠ suit. Bid NT with a stopper in the 4th suit Show delayed 3-card support for Responder's (likely) 5-card ♣ or ♦ suit. Failing any of the above, Responder rebids his own 5-card suit Responder must bid again if 4th suit forcing Opener makes another bid ! The 4th Suit bidder may Pass at any time.