In Mac OS Sierra, you may have become annoyed that your Mac inserts a period, as if it assumes a user is at the end of a sentence when pressing the spacebar twice. Users may be working with data or other formatting, an often don't want the period inserted.
This is another example where Apple changes behavior of the Mac operating system without notification for end users.
Fortunately the automatic insertion of a period can be disabled.
Go to ...
Apple Menu >> System Preferences ...
If necessary, select the LEFT ARROW or ICON for the front panel for preferences.
Select the Text tab
Uncheck "Add period with double-space"
Close System Preferences