Piquet is widely regarded and variously documented as "the best game ever devised", "the aristocrat of card games for two" and "one of the best and most skillful card games for two players".
Piquet is a game of exchanging cards, declaring points, sequences and sets, and then playing off the cards as tricks. What you exchange will affect what you can declare, and what you declare will provide insight to the cards you hold and therefore affect how your opponent plays to the tricks. Points are accumulated for both declarations and tricks - so a carefully balanced strategy is the key to winning.
A Partie (or game) is comprised of six rounds, three dealt by each player alternately. The non-dealer exchanges one or more cards with those that remain undealt, then the dealer exchanges up to as many as are left. If any cards remain untaken after the exchanges, the dealer may elect that they be displayed face-up for both players to see.
The declaration phase starts with the non-dealer declaring their point - i.e, the longest suit. The player with the better point wins the point declaration. Then sequences (runs of consecutive cards of the same suit) are declared, and finally sets (cards of the same face value). If a player wins a sequence or set declaration, they may declare any other sequences or sets that they hold.
After the declarations are complete, the cards are played as tricks with points awarded for leading and for capturing the lead.
Piquet is an excellent game for developing general card playing skills because it includes declaring, melding and trick playing, the three basic elements that are found in most card games. Also, the ability to remember which cards have been seen and the ability to deduce what the opponent holds are key factors to winning. http://www.bufton.org/meggiesoft/piquet.htm