The ASCII horizontal tab character defaults to 8 columns, which is unfortunate because it's too wide for indenting block structured languages (at least to most people's taste, acknowledging Linus Torvalds as a counterexample) and too narrow for columnar text such as assembly language. Looking for how this came about, I found https://en.wikipedia.org/wiki/Tab_key#Tab_characters
Originally, printer mechanisms used mechanical tab stops to indicate where the tabs went. This was done horizontally with movable metal prongs in a row, and vertically with a loop of mylar or other tape the length of a page with holes punched in it to indicate the tab stops. These were manually set to match the pre-printed forms that were loaded into the printer. In practice, settable tab stops were rather quickly replaced with fixed tab stops, de facto standardized at every multiple of 8 characters horizontally, and every six lines vertically. A printing program could send zero or more tabs to get to the closest tab stop above and left of where it wanted to print, then send line feeds and spaces to get to the final location. Tab characters simply became a form of data compression.
Despite five characters were the typical paragraph indentation on typewriters at that time, the horizontal tab size of eight evolved because as a power of two it was easier to calculate with the limited digital electronics available...
When did this come about, that the fixed tab stops were de facto standardized at 8 characters horizontally?