Line Numbers and Style
It was a matter of programming style, if not outright necessity, in these languages to leave gaps between successive line numbers—i.e., a programmer would use the sequence (10, 20, 30, …) rather than (1, 2, 3, …). This permitted the programmer to insert a line of code at a later time. For example, if a line of code between lines 20 and 30 was left out, the programmer might insert the forgotten line at line number 25. If no gaps were left in the numbering, the programmer would be required to renumber line 3 and all subsequent lines in order to insert the new line after line 2. Of course, if the programmer needed to insert more than nine additional lines, renumbering would be required even with the sparser numbering. However, this renumbering would be limited to renumbering only 1 line per ten lines added; when the programmer finds he needs to add a line between 29 and 30, only line 30 would need to be renumbered and line 40 could be left unchanged.
Some BASICs had a RENUM command, which typically would go through the program (or a specified portion of it), reassigning line numbers in equal increments. It would also renumber all references to those line numbers so they would continue to work properly.
In a large program containing subroutines, each subroutine would usually start at a line number sufficiently large to leave room for expansion of the main program (and previous subroutines). For example, subroutines might begin at lines 10000, 20000, 30000, etc.
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—Ralph Waldo Emerson (1803–1882)
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