Herzog is played like a cross between Dominoes and Rummy. Players lay down one card at a time (like domino tiles) which must play off a previously played card according to the rules of Rummy. For example, a Two of Hearts can be played off a Two of Spades (same kind). A Two of Hearts can also be played off an Ace of Hearts (same suit in sequence).
To play a card from your hand, select it by tapping it. The border of the card will turn red. Then, tap in the field where you want to play it. Herzog will follow a short moment later by playing a card next to yours (or onto the discard pile if he has no playable cards). In Pass Play a prompt will appear asking you to pass your device to the other player.
If no card in your hand can be played onto the field, then you must place a card onto the discard pile. However, if you have a card that will play onto the field you are not required to play it. You may discard instead. To discard, tap a card in your hand and then tap the discard pile.
Both you and your opponent may play the card from the discard pile onto the field. To play the discard onto the field, tap it (the card border will turn red), then tap in the field where you want to play it.
You can also pick up the discard into your hand (just like in regular rummy). To do this, tap the discard first (the card border will turn red), then tap on the card in your hand that you wish to exchange. If you want to unselect a card simply tap it again.