Fixed Format

Prior to Fortran 90, Fortran code followed a well-defined fixed format. Comments are indicated with a 'C' in the first column, columns 2-5 were reserved for an optional numerical statement label, a non-blank character in column 6 indicated the current line was a continuation from the previous one, and columns 7 through 72 were available for program statements. Columns 73 through 80 were ignored and often contained line sequence numbers. Blank lines were not allowed. This rigid formatting was the result of Fortran being developed in the era of batch computing and punched card input. The sequence number was used in the case a program 'deck' was dropped; program order could be recovered if the punch cards were placed in a card reader and sorted on columns 73-80. Compiler vendors offered extensions to this formatting, but it was rarely portable (for example, interpreting tab characters as 6 spaces.)

Note that while column position was significant, white space was not. The following program illustrates legal use of white space in fixed-format Fortran:

      PROGRAM Z
C2345678901234567890
      GOTO11
   11 CONTINUE
      GO TO 780
 780  CONTINUE
      G OTO3 60
 360  CONTINUE
      STOP
      END

While this code is technically legal, it is strongly encouraged to use white space to separate keywords, labels, and data to maintain readability

Fortran was developed before the standardization of the ASCII character set and traditionally Fortran code has been written in all-caps. Variable names were limited to six characters, but this was often extended by compiler vendors.

Free Format

Originally Fortran required all statements to be in ALL CAPS, and in fact the language itself was called FORTRAN (in caps). It remains a bit customary, though completely unnecessary, to put Fortran commands in ALL CAPS.

 PROGRAM test
 IMPLICIT NONE
 INTEGER four
 four = 4
 WRITE (*,*) four
 END PROGRAM

Whitespace and empty lines usually won't matter in F90 or above, but in rare cases errors will pop up. However, unlike many other languages such as C, C++, and Java, the line delimiter ';' is optional, so each line of code may stay on its own line! Note that the use of the command separator character ';' is strongly discouraged.