Apple today released WatchKit to developers, letting them develop apps for the Apple Watch. The developer kit gives us a lot of new information about the Apple Watch that the company didn’t share when the device was previewed.

Apple Watch’s Human Interface Guidelines reveal that the two sizes of the Apple Watch run at two different resolutions. The 42 mm version has a resolution of 312 x 390 pixels while the smaller 38 mm version has a resolution of 272 x 340 pixels.

The difference in resolution isn’t a lot, so Apple suggests developers use the same image resources for both sizes if it looks good, else use different asset sizes. The difference in the icon sizes is shown in this table:

The circular home screen icon is 172 x 172 pixels on the smaller variant, and 196 pixels on the larger sized Apple Watch. The Apple Watch runs in 2x mode, and requires 2x images in app bundles.

The Apple Watch typeface is called San Francisco. Apple Watch apps have two components, an interface skeleton on the Apple Watch, and a separate extension that drives this interface, and runs on the iPhone in the background. This is presumably to make the iPhone do all the heavy lifting, and save battery on the Apple Watch. WatchKit is compatible with iOS 8.2 beta, also released today.

For the entire Apple Watch Human Interface Guidelines document, head to this link.