Underline or italics? In our manuscripts, we use one or the other, never both. They mean the exact same thing.
In the days of older typewriters, writers had no way to italicize certain titles, foreign and special words, to show emphasis, or the names of ships, aircraft, and bridges in manuscripts. We backspaced and tapped the underline key to show where we wanted italics.
When I began college, I purchased an innovative IBM Selectronic typewriter which allowed the exchange to a ball with all-italics font. It was time consuming, but no more underlines in papers or manuscripts.
In grad school, I upgraded to a TRS 80 computer with a daisy wheel printer. State of the art. Yep, but I still had to stop and exchange to a daisy with different petals to print italics on the page.
With our modern word-processor, italics are just a couple of clicks away.
Each publisher has a guideline for manuscript fonts. It’s best to check, but many prefer Times New Roman 12 or Courier New 12.
T-Roman seems to handle italics font without confusion where it starts and stops.
With Courier font, many manuscript readers prefer we underline where we want italics.