Fortran User's Guide

Tabs in the Source

f90 allows the tab character in fixed-form source and in free-form source. Standard Fortran 90 does not allow tabs.

The tab character is not converted to a blank, so the visual placement of tabbed statements depends on the utility you use to edit or display text.

Fixed-Form Source

Free-Form Source

f90 treats a tab and a blank character as equivalent, except in literal strings.