Punctuation has usually been ignored by researchers in computational linguistics over the years. Recently, it has been realized that a true understanding of written language will be impossible if punctuation marks are not taken into account. This paper contains the details of a computer-aided exercise to investigate English punctuation practice for the special case of comma (the most significant punctuation mark) in a parsed corpus. The study classifies the various "structural" uses of the comma according to the syntax-patterns in which a comma occurs. The corpus (Penn Treebank) consists of syntactically annotated sentences with no part-of-speech tag information about the individual words.