Flipping the anchor and head
Every selection has two ends. The anchor is where the selection started, and the head is where your cursor is. Motions move the head, which is why a selection grows in only one direction.
Alt-; swaps the two ends, putting the cursor on the anchor. Remember this
one, because it’s really handy! It means the position you started a
selection from isn’t fixed.
For example, if you build a selection and realize the start is in the wrong
place, you don’t have to collapse it and begin again. Press Alt-;, move
the cursor to fix the start, and press Alt-; again to carry on from the
other end.
Work through the checklist below — you’ll see that it’s possible to adjust a selection in both directions.