For those who have become accustomed to typing on an iPhone or iPad, macOS 10.12 Sierra has two new options to make the experience similar on the Mac. Open System Preferences > Keyboard > Text and you’ll see two new checkboxes: “Capitalize words automatically” and “Add period with double-space.” The first automatically capitalizes the word that follows sentence-ending punctuation. Happily, it’s smart enough not to capitalize words like iPhone and iPad. The second option inserts a period if you press the Space bar twice after typing a word.
Source: TidBITS