
The flat sides allow the letters to be laid out with more space between them, thereby making the text more legible at small sizes, which is particularly important for the Apple Watch. The main difference is that the sides of letters with round shapes, such as o, e, and s, are round in SF, whereas they are flat in SF Compact. Note: SF has the codename SFNS in macOS and SFUI in iOS, regardless the official name. In macOS High Sierra and iOS 11, SF UI was succeeded by SF Pro. The popular graphic design service has its own app for the iPhone with over 1,300 fonts for free. The next year at WWDC, Apple released the watchOS font as SF Compact and at the same time introduced SF UI (generally called SF) for OS X El Capitan and iOS 9. San Francisco was first introduced in watchOS only.


Several other variants exist for internal use by Apple. The San Francisco typeface has three main variants: SF for macOS, iOS, and iPadOS SF Compact for watchOS and SF Mono for the Terminal, Console, and Xcode applications.
