Forty Thieves is a patience game played with two full packs of cards. At the start of the game forty cards are dealt on the eight 'bases' along the top of the window. The object of the game is to place all the cards onto the eight 'foundations'. The foundations are built starting with the ace and adding cards of the same suit up to the king. Cards are dealt from the pack and placed on the discard pile. Cards may be moved from the discard pile or one of the bases to a base or a foundation. Only one card can be moved at a time. Cards can only be placed on a base if the top card of the base is of the same suit and is one higher in pip value or the base is empty e.g. the eight of spades can only be placed on top of the nine of spades.

When the mouse cursor is over a card which can be moved it changes to the 'hand' cursor. The card can then be moved by double clicking the left button.

The mouse cursor also changes to a hand when a card is dragged by placing the cursor over the card and holding down the left button. This feature can be enabled and disabled by selecting the 'Helping hand' option from the Edit menu.

The 'foundations' are the eight piles of cards down the left side of the window. When the game starts these piles are empty. The object of the game is to place all the cards on the foundations. An ace can be placed on any empty foundation. Other cards can only be placed on a foundation if the top card is of the same suit and is one lower in pip value e.g. the three of clubs can be placed on the four of clubs.

The 'bases' are the ten piles of cards along the top of the window. At the start of the game four cards are dealt face up on each of the bases. A card can be added to a base if the base is empty or if the top card is of the same suit and is one higher in pip value e.g. the queen of hearts can be placed on the king of hearts The top card of a base can be moved onto another base or a foundation.

Cards can be only moved one at a time. The top card of the pack can be dealt onto discard pile by placing the mouse cursor over the pack and pressing the left button. The number of cards remaining is displayed to the right of the pack.

Cards can be moved from the discard pile or the bases either by 'double-clicking' or by dragging. If the left button is double-clicked when the mouse cursor is over a card and it can move to another pile, it will do so. This is a quick way of moving cards when their destination is unambiguous.

A card can be dragged by placing the mouse cursor over the card and holding down the left button. The card will follow the mouse cursor until the left button is released. If the card is over a pile on which it can be placed it will be added to that pile, otherwise it will be returned to the pile from which it was dragged.

One point is scored for every card that is placed on a foundation. Since there are two packs of 52 cards the maximum score is 104. A record is kept of the number of games played, the number of games won, the current score and the average score. This information is displayed at bottom right of the window and is stored on disk between games. A game is deemed to have started if the cards have been dealt and any card has been moved. If the game is abandoned before it is finished (i.e. by starting a new game or closing window) it counts as a lost game. New players can be added by selecting the 'Player' option from the Game menu. A summary of players' scores can be displayed by selecting the 'Scores...' option from the Game menu.

All moves are recorded and can be undone. To undo a move select the undo menu item from the Edit menu. A quicker way of undoing is to press the right mouse button (it doesn't matter where the mouse cursor is). Right button undo can be enabled and disabled by selecting the 'Right button undo' option from the Edit menu. Pressing the right mouse button with the control key pressed re-does a previously undone move.

An empty base or two is very useful as it gives the opportunity to unscramble other bases. Try not to build onto kings which obscure valuable cards as it will be difficult to get to them later. The undo facility is very useful for going back and using 'hindsight'.