A while ago, Microsoft released Windows RT for devices with ARM chips. Windows RT wasn’t successful for the company, and existing Windows RT devices didn’t receive the Windows 10 upgrade either. Now, it seems like Microsoft isn’t done with Windows on ARM chips.
A documentation on MSDN reveals Windows 10 for desktop editions (Home, Pro, Enterprise, and Education) ARM. In addition to this, the same documentation also lists Windows 10 Mobile x86 and Windows 10 Mobile ARM64, which could be used in the rumored Surface Phone.