Epson FX Series User Manual page 87

You can turn Underline Mode off with:
CHR$(27)"-"CHR$(0) or
Enter and RUN this program to see what FX underlining looks like:
NEW
20 LPRINT CHR$(27)"-1UNDERLINING IS SIMPLE";
40
LPRINT CHR$(27)"-0 TO TURN ON/OFF"
You can underline virtually anything you want-even a series of
blank spaces. Try adding this line, typing in the spaces where we have
used characters:
Being able to underline a blank space really helps when you want to
make a form that has lines to be filled in later with signatures or data.
The FX Underline Mode can even underline leading spaces with
BASIC TABS-although it doesn't work with the FX's internal tabs
that are set with ESCape "D" (see Chapter 9).
Underline Mode does not use the FX's underline character. Since the
underline character is itself only five dots wide, it would not print in
the spaces that separate the text characters. See this by printing a row
of underline characters above the text in the current program:
10 FOR X=1 TO 42: LPRINT CHR$(95);: NEXT X: LPRINT
When the FX is in Underline Mode, it is simply using the ninth pin to
underscore the entire passage.
How far under a character its underline appears depends on the
model of FX that is printing. The FX-100 prints a character and its
underline at the same time, and the underline occurs on the ninth row
of the nine-row character matrix. This means that the underline prints
over the bottom row of the descender for a few lowercase letters, such
as y and g.
70
CHR$(27)"-0"