It's pretty pedantic, and this is not true in all dialects and forms of English[0] - as evidenced by the fact that you have at least three other comments replyi

It's pretty pedantic, and this is not true in all dialects and forms of English[0] - as evidenced by the fact that you have at least three other comments replying with different, conflicting answers to this question! In any case, the comma basically indicates that the two words serve the 'same function', and that either one could be removed (along with the comma) while leaving the parsed structure of the sentence intact. That doesn't exactly mean that the meaning would be intact (you're removing a word, after all), but the way that you'd parse the rest of the sentence must remain identical. At a practical level, this means that 'noncanonical Gnostic texts' translates to 'Gnostic texts which are also noncanonical'. 'noncanonical, Gnostic texts' means more or less the same thing, but also implies that there is a connection between the fact that they are noncanonical and the fact that they are Gnostic. This may be because the two are outright synonyms, or simply because they are often connected. For example: > heavy, red rocks > heavy red rocks It'd be weird to say 'heavy, red rocks', unless in context you had some reason to connect those two (such as those being identical sets, if all red rocks are heavy)[1]. Instead, you'd say that the red rocks are heavy - in other words, "the (heavy (red rocks))". Also, note what I did with the opening paragraph of this response. 'different, conflicting answers' (because the answers are different, and they are conflicting, and the way I am using the word 'different' is connected to the fact that they are (partly) 'conflicting')[2]. [0] ie, this is strictly limited to the stylistic rules of journals of record, such as the New York Times, and not necessarily applicable to the way those same writers/editors would write an email. Curiously, I believe this is true both in American English and in formal British English. [1] 'The machine picks up all the heavy, red rocks and leaves behind the light, blue rocks' vs. 'The machine picks up all the heavy red rocks and leaves behind all the light red rocks and the blue rocks (whether they are heavy or light)'. [2] Though the response which directly contradicted mine has since been deleted, but, uh, ignore that fact.