Developers making apps for the Apple Watch have a new rule: You can’t make a watch app.

Apple had already been rejecting time-telling apps from the Apple Watch App Store, but it finally codified that policy into a formal rule, 9to5mac reports.

What this means in practice is that Apple Watch developers can’t sneak new custom watch faces onto the device. Unlike many competitors, the Apple Watch doesn’t support watch faces made by anybody but Apple. There is a chance, however, that Apple could open up that possibility later this year.

The Apple Watch went on sale April 24, though most models are currently backordered until June. The device, Apple’s first wearable, starts at $349 and goes up to $10,000 and above depending on options. It comes with ten watch face options, each with some customizable elements.

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Write to Alex Fitzpatrick at alex.fitzpatrick@time.com.