To take a screenshot of a selected area, first press ⌘⇧4 and then drag a selection. But it gets better! During the selection, you can press and hold ⇧ to lock the selection area in a certain dimension (pressing ⇧ and dragging left/right will lock the height of the box while up/down will lock the width of the box).
Useful Mac continues to be, well, obviously, useful[^-361149610]. This time with information about keyboard-shortcuts for taking screenshots.
There’s a lot more to learn in the article. Very useful.