Video tape question

On a video cassette there are evenly spaced hash marks on both sides of the reel. Are they some sort of measuring device and does each space equal a certain length of time? Just curious.

At one time, Scotch brand tapes provided a sticker you would place on the tape to tell you the approximate time left (based on tape speed). Without this sticker, it would be difficult to judge the amount of tape left without experimenting.