Most of us refer to these as the "punch-card" type, as the casino punches the combination of holes (or slots) in the card to signify the correct account. When these types of cards (which are still in use at some locations) are placed in a card reader, the mechanism in the box shines a light down, and reads where the holes are from the patterns of light (holes) and dark (no holes.) Each player has a different combination. This system is based on the old computer punch card readers, which is another reason to refer to them as "punch-cards." The more modern cards with magnetic strips, have the same info magnetically on the card, and has room for much more data.