Why did typewriters go ding just before reaching the end of a line?

Oh my, this question takes me back!

Before the days of word-processors, you typed straight onto the paper with metal keys and an inked ribbon.There was no 'return' key and no automatic justification and you had to move the carriage yourself to start a new line. The 'ding' was to let you know that you were close to the end of the line. If memory serves me correctly it was about 5 spaces from the end, so that when you were touch-typing and not looking at the page, you had advance warning of when to shift the carriage back for the next line.

As for the "how far" question, I suppose it differed from model to model, but I seem to recall it was about 3 to 5 spaces before the end.

Now here's another blast from the past: Remember how we used to type the square bracket characters like [ and ] that there are keys for on any present-day keyboard? Slash, underscore, manually roll the paper backwards one line, backspace, underscore, manually roll the paper back to the original line. Great fun if you're typing a scholary or scientific article full of brackets!

So you know when to slide it back

Back in the days where text processing was done straight on the media (paper in this case), there's no virtual guidelines pre-printed, so a "ding" sound has to be generated to warn typist that they are a few characters near the physical border of media.

In such case, if you're typing a super long words (no full justifications too), you know when to "dash-break" it, or where you would like to insert a "manual line break" by taping on the handle and pushing the barrel all the way left (left justification and no-auto line-breaks is by default the one and only option).

To let you know there's no more typing available on that line. When it rings you acant type any more.