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Identifier of a sequence of characters in a text file
In computing, a line number is a method used to specify a particular sequence of characters in a text file. The most common method of assigning numbers to lines is to assign every line a unique number, starting at 1 for the first line, and incrementing by 1 for each successive line. In the C programming language the line number of a source code line is one greater than the number of new line characters read or introduced up to that point. Programmers could also assign line numbers to statements in older programming languages, such as Fortran, JOSS, and BASIC. In Fortran, not every statement needed a line number, and line numbers did not have to be in sequential order. The purpose of line numbers was for branching and for reference by formatting statements. Both JOSS and BASIC made line numbers a required element of syntax. The primary reason for this is that most operating systems at the time lacked interactive text editors; since the programmer's interface was usually limited to a line editor, line numbers provided a mechanism by which specific lines in the source code could be referenced for editing, and by which the programmer could insert a new line at a specific point. Line numbers also provided a convenient means of distinguishing between code to be entered into the program and direct mode commands to be executed immediately when entered by the user (which do not have line numbers). Largely due to the prevalence of interactive text editing in modern operating systems, line numbers are not a feature of most programming languages, even modern Fortran and Basic.

In COBOL, line numbers were specified in the first six characters (the sequence number area) of punched cards. This was originally used for facilitating mechanical card sorting to assure intended program code sequence after manual handling. The line numbers were actually ignored by the compiler.

In 1962, DOPE (Dartmouth Oversimplified Programming Experiment) became one of the first programming languages to require a line number for every statement and to use sequential ordering of line numbers. Line numbers were specified as destinations for two commands, C (Compare operation, an arithmetic IF) and T (To operation, a GO TO).

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 they need 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 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.

If a programmer introduces a syntax error into a program, the compiler (or interpreter) will inform the programmer that the attempt to compile (or execute) failed at the given line number. This simplifies the job of finding the error immensely for the programmer. The use of line numbers to describe the location of errors remains standard in modern programming tools, even though line numbers are never required to be manually specified. It is a simple matter for a program to count the newlines in a source file and display an automatically generated line number as the location of the error. In IDEs such as Microsoft Visual Studio, Eclipse or Xcode, in which the compiler is usually integrated with the text editor, the programmer can even double click on an error and be taken directly to the line containing that error.