Since the birth of the game Magic: The Gathering in 1994, trading card games have been steadily growing in popularity over the years. While many have different goals and different odds, there are some key concepts which are common to many; one of these is the concept of tapping cards.

While not universal, the concept of tapping is common to many trading card games, such as Magic: The Gathering, Legend of the Five Rings, and Netrunner.

In these games, many cards stay on the table when played. These cards then tend to fall under different states of play, one of which is untapped, and another being tapped; cards that are tapped are generally at a 90 degrees angle to represent this, although some games may use more esoteric methods- for example, the game Arcadia instead turns the card over so that the back of the card is face up.

Mechanics and Concept
Tapping is generally used to represent exhaustion of a resource; a card that is tapped cannot be tapped to pay for costs. Many cards have abilities on them which involve tapping themselves or other cards to pay for costs, and others tap cards as part of their resolution. When a card is tapped, it is rotated 90 degrees to show it is in the tapped state of play.

Cards that have been tapped tend to untap at the beginning of a player's next turn, although other cards may untap them before then, and the effects of some cards may keep them tapped longer.

When a card is referring to tapping itself as a cost, it will do so in a manner listed within the game rules. For example, Magic: the Gathering has a symbol of a bent arrow in a small black rectangle, which is read as "Tap this card to have the following effect"; Legend of the Five Rings, on the other hand, uses the word "Bow" to mean "Tap"- as in "Bow the (cardname) to (have this effect)."

An example of how Magic: The Gathering (MTG) deals with Tapping is on the card Archivist.
"(Tap Symbol): Draw a card."
This translates as follows: While Archivist is untapped, you can tap it to draw card, but you can't if it's tapped.

Bearing in mind that the word "Bow" is Legend of the Five Rings (L5R) parlance for "Tap", an example of the way L5R phrases tapping is on the card Diamond Mine:
"Bow the Diamond Mine to produce 5 Gold."
This is more straightforward: As long as the card is untapped, you can tap it to produce five gold.

Other cards will ask you to tap other cards as part of their cost. MTG normally makes note in this instance that you require an untapped card, whereas L5R sees this as already implied; the difference is seen between the cards Azami, Lady of Scrolls and Simple Huts.

Azami, Lady of Scrolls reads:
"Tap an untapped Wizard you control: Draw a card."

Simple Huts reads:
"Bow Simple Huts and one of your Farms to produce 5 Gold."

The difference is simple. L5R assumes that both cards are unbowed/untapped before bowing them, whereas MTG does not; in MTG, only the tap symbol implies that a card must be untapped before tapping it; if Azami had read "Tap a Wizard you control: Draw a card", then under current MTG rules, you would be able to tap one Wizard any number of times and draw that may cards!

Other card effects may tap or untap other cards as part of their effect, although in most games all cards untap at the beginning of their controller's turns. For example, there's a Magic card called Seeker of Skybreak that reads the following:
"(Tap Symbol): Untap target creature."
This card can, therefore, be tapped itself to untap any one tapped creature card.

An opposite number to the Seeker is a card called Sand Squid, which reads:
"You may choose not to untap Sand Squid during your untap step.
(Tap Symbol): Tap target creature. That creature does not untap during its controller's untap step as long as Sand Squid remains tapped."
Sand Squid may be tapped to tap any creature, whether it's tapped or not; you may want to tap a creature that's already tapped in this way because you want to keep it tapped.

A common sticking point behind this is on MTG cards such as Bringer of the Red Dawn, which reads:
"At the beginning of your upkeep (trans. 'once per turn, after you've untapped everything'), you may untap target creature and gain control of it until end of turn."
The problem with this is that some newer players assume that the creature has to be tapped to allow Bringer of the Red Dawn's controller to take control of it. this isn't true; as it doesn't say "untap target tapped creature and...", you can target any creature, tapped or not.

The Etymology
The term "Tapping" was invented in 1994 by Richard Garfield to manage resources in the game of Magic: the Gathering. It was originally used on a type of card called "lands", cards which were used to play other cards by adding a resource called "Mana" to an imaginary pool. As he needed this Mana to only be supplied once per turn, he came upon the elegant idea of showing the card had been used by turning it on it's side.

The process of gathering the mana from lands is represented in the game as the mage that the player represents connecting with the land to draw off mana from it, hence the name; the mage taps into the mana of the land.

Euphemisms
Legend of the Five Rings isn't the only card game to use a different name to tapping; a lot of cards out there use terms for tapping other than the one used in Magic. A non-exhaustive list of them follows; if you know of any more games which use tapping, please /msg me and I shall add them to the list.


+----------------------------------------+
| A Game of Thrones            | Kneel   |
| Aliens Versus Predator       | Rotate  |
| Arcadia                      | Exhaust |
| Babylon 5                    | Rotate  |
| Battletech                   | Tap     |
| Call of Cthulhu CCG          | Exhaust |
| Doomtown                     | Boot    |
| Dune                         | Engage  |
| Firestorm                    | Turn    |
| Heresy: Kingdom Come         | Open**  |
| Legend of the Five Rings     | Bow     |
| Legend of the Burning Sands  | Bow     |
| Magic: The Gathering         | Tap     |
| Middle Earth                 | Tap     |
| Netrunner                    | Tap     |
| Ophidian 2350                | Set     |
| Seventh Sea                  | Tack    |
| Shadowfist                   | Turn    |
| Shadowrun                    | Turn    |
| Star Wars: Trading Card Game | Tap     |
| Tempest of the Gods          | Drain   |
| Vampire: Eternal Struggle    | Tap     |
| Warhammer 40,000 CCG         | Lock    |
+----------------------------------------+

**According to hapax, whereas "opening" is the Heresy equivalent to tapping, one can also "strike" some cards; when a card is stricken, it is turned over. At the beginning of each player's turn, all cards that player controls that are stricken are flipped over and become become opened, and all cards that are opened become ready-- meaning a "stricken" card is effectively tapped twice.