21.13.2.2 Line Numbers and Continuation

If a number is the first non-whitespace in the line, it is assumed to be a line number and is moved to columns 0 through 4. (Columns are always counted from 0 in SXEmacs.) If the text on the line starts with the conventional Fortran continuation marker ‘$’, it is moved to column 5. If the text begins with any non whitespace character in column 5, it is assumed to be an unconventional continuation marker and remains in column 5.

Line numbers of four digits or less are normally indented one space. This amount is controlled by the variable fortran-line-number-indent, which is the maximum indentation a line number can have. Line numbers are indented to right-justify them to end in column 4 unless that would require more than the maximum indentation. The default value of the variable is 1.

Simply inserting a line number is enough to indent it according to these rules. As each digit is inserted, the indentation is recomputed. To turn off this feature, set the variable fortran-electric-line-number to nil. Then inserting line numbers is like inserting anything else.