Play Coins are a currency introduced on the Nintendo 3DS, earned primarily by gaining steps on the pedometer while the system is asleep. They are used more widely as a feature of the Display, where they can be earned in other ways in apps, games, and achievements. Play Coins can be spent in several ways and on a myriad of things; their usage is rarely consistent between games.
Each Play Coin appears as a bright yellow coin of indeterminate size, with a large circle and an icon of the current system engraved onto the front. Play Coins transferred from the Nintendo 3DS to the Display, however, retain their 3DS-shaped engraving.
A Play Coin can be earned on either system by getting 100 steps. On the Nintendo 3DS, only 10 Play Coins could be earned this way per day. Play Coins can also be awarded on the Display in games and apps. Developers are even able to set up an achievement system, such that different amounts of Play Coins are earned when arbitrary conditions are met for the first time.
The upcoming app Play Mint is an idle game that will award progressively more Play Coins as gameplay advances, to an extent. It will also allow Play Coins to be transferred between the 3DS and the Display.