Mountain is played with two decks of standard playing cards plus optional jokers.
In the solitaire game the decks are shuffled together and 12 cards are dealt in a row to form the base of the mountain. The player takes one card from the deck at a time and places it in the discard pile; the top card of the discard stack is available to play on the mountain.
A card on one level is placed above two cards on the next lower level. If the cards are different ranks and suits the new card must match the rank of one and the suit of the other; otherwise it must match what they have in common. (If the cards are identical any card can be played on them.)
When the deck runs out and all plays have been made, cards score based on what level they are on.
In the two-player game one player has a deck of red cards and the other black. The base is made up of alternating red and black cards and players draw cards into their hands, from which they may play any number of cards. The winner is the player with the most cards on the top level, but if a player can complete an entire level by themselves they win immediately.