A comment starts with two adjacent hyphens and extends up to the end of the line. A comment can appear on any line of a program. The presence or absence of comments has no influence on whether a program is legal or illegal. Furthermore, comments do not influence the effect of a program; their sole purpose is the enlightenment of the human reader.
-- the last sentence above echoes the Algol 68 report
end; -- processing of LINE is complete
-- a long comment may be split onto -- two or more consecutive lines
---------------- the first two hyphens start the comment
Horizontal tabulation can be used in comments, after the double hyphen, and is equivalent to one or more spaces (see 2.2).
References: end of a line, illegal, legal, space character.