Four-Player Chess/Common openings/1. h3
King's Pawn Opening, 1.h3
This opening, sometimes also called "dark-squared opening", is by far the most popular opening for Red, especially among more experienced players. Red opens up the dark-squared g1-n8 and a9-i1 diagonals for the queen and bishop, respectively, eyeing both Green and Blue. It stops Blue from playing c8 or d8 and opening the dark-squared a9-i1 and a7-h14 diagonals, since that would allow a double attack on Blue's queen and bishop after Yellow also opens the a7-h14 diagonal with the move g12 or g11. If Blue and Green are ignorant and both open the dark-squared diagonals of their queen and bishop as well and Yellow does the same, then Red and Yellow can win a bishop after all queens are exchanged. It could even lead to Fool's mate, if Blue and Green are really careless.
Blue has to decide whether he also wants to open on the dark squares, open on the light squares or play another move. Since Red opened up the dark-squared diagonal, Blue cannot play c8 immediately. If he would, yellow would play g12 (or g11) and then both Blue's bishop and queen are under attack. However, Blue can prepare the move c8 by playing the move d6 first, closing off the a9-i1 diagonal. Alternatively, Blue can choose to play c7 and open on the light squares. Other moves are possible too, but these two are the most commonly played.
Blue closes off the diagonal, such that c8 can be played next. The pawn on d6 is actually en prise and can be taken by Red's bishop, but Red is ill-advised to do so, because Blue and Green can gain a tempo by attacking it and Red will have to move it again. Development is of key importance and pawns are not that valuable in the opening phase of the game. Also, taking the pawn makes it easier for Blue to develop the rook(s) at a later stage.
Blue does the opposite of Red and opens up the light-squared a6-i14 and a8-h1 diagonals. Yellow usually responds with h12 or h11, immediately creating tension between the bishops with the idea that Red can play Qk5 or e3 to attack the potential weakness on b5 in case the bishops are exchanged. A common idea in this opening is for Green to open with l10 and for Blue to reply with c6 to Red's Qk5, with the idea that Green can play l7 next, forcing Red to trade queens.