Yes, you can be signed in on multiple devices with the same account, both iOS (iPhone, iPad) and Android (Samsung Galaxy, Pixel etc.) 

Your progress state and vocab list for a given story are linked to the account, so that information will be up-to-date on any device you use.

We do recommend you only use one device at a time, allowing it to sync your progress to the cloud (which happens automatically if you're online) before you start studying on another device.

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