Visual mode and the range operator

By now, we’ve probably seen this thing a few times: '<,'>

What does that mean?!

We can decipher that now. First, understand that Vim uses the < and > marks to represent the start and end of a visual selection respectively. If you highlight a range of lines, the < mark will be placed at the start of your visual selection and the > mark the end.

Now recall that 't is an address specifier that specifies the line of mark t.

Putting these two facts together, we understand that '<,'> is a range that represents the current (or previous) visual selection! That’s why, if you’re in visual mode and you type :, Vim will pre-populate the command line with '<,'>. It’s because most of the time, if you’re issuing an Ex command from visual mode, you want to act over the selected region.

Let’s try to convince ourselves of this in the editor. Visually select the first paragraph, then re-enter normal mode. Then type :'<,'>d manually into the command line. The paragraph gets deleted, even though we’re no longer in visual mode any more! That’s because Vim preserves the '< and '> marks, even if you’re no longer in visual mode.