Statements within a particular procedure (or in the main session) are recognized in levels, determined by the nesting of conditional or repetition statements and the nesting of procedures.

The top (interactive) level is level 0; statements within conditional/repetition statements are level 1, or level 2 if doubly nested, et cetera. The first procedure is level 5; statements within conditional/repetition statements within a procedure are level 6, et cetera. The nth nested procedure is level .

The setting of the printlevel environment variable causes the display of the results of all statements executed up to the level indicated by printlevel. Initially, printlevel is set to 1.

Higher values of printlevel will cause more information to be displayed; negative values will cause no information to be displayed.

When execution errors are encountered and  a summary of calling routines, such as provided by tracelast, is also provided.

If printlevel is assigned an integer less than 0, printed output from all commands is suppressed.

If an attempt is made to assign a non-integer to printlevel, an error is generated and the value of printlevel is not changed.

For debugging purposes,  is not uncommon.

For more comprehensive debugging facilities, see debugger.