The Useful Line Indicator
The metal bar often with either grooves or embossed with white lines or in later century models the clear plastic Perspex card guides often with pre-printed white or red lines and/or hash marks on either side of the a typing point are called the line indicator. The “line” it suggests is the potential or actual line of type across the page and is often called the typing line. If it’s properly adjusted it should be set to the base line of the characters you’re typing and also fit the characters either between them or point at the center of the characters. This way if you remove a page and then need to type on it again, you can very precisely level and align the typing line and the characters themselves so that no one will know the page was typed in one or multiple passes. It’s also useful if you accidentally use the permanent variable in the platen and shift the regularity of your lines, then you can properly re-align them and continue typing.
Careful use of the line indicator and the character spacing hash marks also allows one to very precisely align pre-printed forms for filling in data either on blank lines or within pre-printed boxes. As an example, you can find a well-aligned version of a baseball scorecard I typed back in March. It relies on using the correct pitch typewriter and aligning the home plate in the top center of the scorecard carefully designed by Lou Spirito. If you compare them, my alignment is more careful than Tom Hanks’ “this is maybe close enough” alignment, which can show how useful the line indicator can be for professional looking typed documents.
The character spacing on the line indicator rule can allow one to also guestimate where they need to either end a word or hyphenate it when they hear the bell at the end of the line. Later machines often had 5-10 spaces on their line indicators to allow just this sort of guestimate as this was often the number of spaces available before hitting the right margin after the bell.

On some models, the line indicator may have a V-shaped groove or small hole cut into it just below the top of the typing line. This is meant to allow the operator to insert a pencil or pen into that space and then using the carriage release to draw a straight horizontal line across the page. Doing the same thing, but with the platen knobs will allow one to create vertical lines on their pages quickly.
On old typewriters with poor or hardened rubber on the platen and rollers, which can allow the paper to slip a bit, or be inserted at a slight angle or on typewriters which don’t have paper guides, one can also use the line indicator to ensure the paper is level when inserted.
As a subtle paper and type alignment hint, most pica and elite machines will lay down type at six lines per inch, so one can use this fact along with the line setting of the carriage return to align the start of their page on either the fourth or the seventh single spaced line to give the page either a 1/2 inch or 1 inch top margin respectively.
Typewriter Rule Registration and Alignment
If you’re repairing or restoring a typewriter you generally want to properly register all the various typing rules on the machine so that they line up with each other. This can include rules on the back of the machine, the paper table, the front of the carriage, the line indicator, and the paper bail. Often one may be a permanent placement and not have adjustment capability in which case the others are aligned to it. Most of the rest of the rules will have oval screw holes which allow them to be shifted left/right to line them up and then screws to tighten them down. Alignment can be effectuated by putting a sheet of paper into the machine squarely and then using both edges of the paper to line up the same markings across all the rules.
The line indicator is one of the few rules that can also be adjusted up/down so that you can type out a line of HHHs or NNNs and adjust both sides so that you can level the line indicator to the baseline of the characters typed across the page to have everything level. It’s this careful alignment in the shop which will allow the professional typist to turn out the best quality material, particularly when it comes to typing out pre-printed forms.
Older machines, particularly ones with manually set tab stops in the back of the machine to allow for accurately setting tabs, will have one or more rules hiding on the back of the machine. Be sure to account for these and adjust them properly as well.

Be aware that on some machines the design might sometimes allow two rules to coincide. As an example, the line indicator rule and the carriage ruler on the Underwood Touch Master 5 are the same rule.

Hopefully now that you know about the subtle art of the line indicator and how it’s used, you’ll be able to better adjust your own typewriter and turn out more sophisticated looking pages.