Game cards are square shaped, with 3DS game cards additionally featuring a small appendage to the right of the game label to prevent them from being inserted into an original DS (though removing the tab and inserting one into the DS reveals that the system will not read 3DS game cards regardless). Game cards are additionally color-coded based on which system they're designed for: DS game cards are dark gray, DSi game cards are white, 3DS game cards are light gray, and Switch game cards are black. While most game cards don't come in unique-color variation, the ones for Pokemon HeartGold and SoulSilver, Pokemon Black and White, and Pokemon Black 2 and White 2 are black with built-in infrared sensors.
The Nintendo 3DS and New 3DS are backwards-compatible with DS & DSi game cards, but the latter two systems are incapable of running 3DS game cards. Additionally, DSi game cards cannot be run on the original DS due to them taking advantage of DSi features that the DS lacks. For the same reason, the original 3DS cannot run game cards designed specifically for the New 3DS.
Despite ROM cartridges having fallen out of fashion for home consoles as a result of the Sony PlayStation's success over the N64, game cards have continuously found success with Nintendo handhelds as a result of their faster loading times (which is especially advantageous due to how handheld games are typically played in short intervals), lower vulnerability to piracy, and reduced system manufacturing costs (optical discs require an optical drive consisting of a spindle and a laser in order to be read and a ventilator to prevent said drive from overheating; cartridges can simply be read with pins attached to the motherboard).