(Corresponds to Section 3.2.3 of ANSI X3.9-1978 FORTRAN 77.)
A continuation line is any line that both
A continuation character is any character of the GNU Fortran character set other than space (<SPC>) or zero (`0') in column 6, or a digit (`0' through `9') in column 7 through 72 of a line that has only spaces to the left of that digit.
The continuation character is ignored as far as the content of the statement is concerned.
The GNU Fortran language places no limit on the number of continuation lines in a statement. In practice, the limit depends on a variety of factors, such as available memory, statement content, and so on, but no GNU Fortran system may impose an arbitrary limit.