An old convention whereby responses of 4♣ or 4♦ to an opening bid of one of a major show a good fit, sound values for game and some interest in a slam, thus releasing the direct raise to four of opener’s major as a pre-emptive action. There are many versions of Swiss. The two most popular were:
Singleton Swiss: 4♣ shows two aces and a singleton, after which 4♦ from partner asks responder to identify the singleton; 4♦ shows two aces without a singleton.
Fruit machine Swiss (Three-Way Swiss or Sussex Swiss): 4♣ shows either two aces and a singleton, or three aces, or two aces and the king of trumps. 4♦ from opener is then a relay requesting clarification. In reply, 4NT shows three aces, a bid of four of the agreed trump suit shows two aces and the trump king, and a new suit shows two aces and a singleton in the suit bid.
These days very few pairs play this convention, preferring instead to use Splinters, often in conjunction with the Jacoby 2NT raise.