Like in other programming languages, comments in Algol 68 programs are intended to document the program and their contents are ignored by the compiler: they are stripped out by the lexer. There are three styles of comments, that differ only by the delimiters used to begin and end the comment.
The first style uses comment to delimit the comment
contents:
comment This program does foo and bar. Written by John Doe. comment
The second style uses co to delimit the comment contents:
if not ok
then co This happens rarely co
abort
fi
The third style uses # to delimit the comment contents:
print (whatever) # XXX remove trace #
Comments of different styles can be nested. Therefore up to three nesting levels is supported, which must be more than enough.