Theories of the comma

Style manuals typically classify comma usage as either heavy or light, but that distinction isn't the fundamental divide, which concerns what to use the comma for, not how much to use it. Two approaches, we'll call them confusion avoidance and clarity enhancement, describe the major philosophies of the comma. The confusion avoiders (avoiders), basing their usage on case-by-case judgment, use commas when omitting them creates ambiguity, confusion, or miscue. Clarity enhancers (enhancers), in contrast, base their comma usage on syntactic units: commas carve sentences at their joints, grouping words belonging to grammatical units grasped whole.

I don't want to exaggerate the differences, which are fewer than the similarities. Both recognize a tradeoff between clarity (heavy punctuation) and concision (light punctuation) and appreciate the need to strike some balance. Both recognize the conditions where a comma is simply wrong, such as separating subject from verb or a verb from its direct object.

The usage patterns diverge in their systematism, but the divergence is one of degree. Enhancers, for example, usually conform to the rule that a comma should precede a coordinating conjunction connecting two independent clauses, whereas avoiders are indifferent to the rule and will more often omit the comma, but exceptions true to the philosophies abound. When the second clause begins with a nonrestrictive phrasal modifier, and punctuating both clause and phrase produces a punctuation surfeit, enhancers might omit the comma before the coordinating conjunction, as here:

This view is espoused by some determinist free-will deniers but as you would expect, the libertarians express the stronger views.

Analogously, avoiders put a comma before the conjunction to avoid sentence-specific ambiguity. Take the sentence,

We owe mass literacy to the printing press, and mass democracy we owe to its progeny, the newspapers.

Omitting the comma before and could be read as crediting both the printing press and mass democracy for mass literacy. To avoid the miscue, avoiders would retain the comma.

The following usage recommendation by an avoider further illustrates the approaches:

Knights wore metal shoes, and gloves called gauntlets.

Point in question, the comma after shoes. Mark Nichol, the recommender, points out that without the comma the sentence says shoes and gloves are called gauntlets. Since I belong to the clarity-enhancement school of thought, I find Mark's practice sloppy and obfuscating; I abjure sticking a comma wherever meaning preservation requires. Separating a group of words to avoid a specific confusion muddies the water elsewhere; here, it submerges the central assertion that knights wore both metal shoes and gloves. The author tries to employ a punctuation mark to do unsuitable work, work only words can perform. The sentence rewritten according to enhancer sensibilities,

Knights wore metal shoes and also gloves called gauntlets.

The contrast illustrates the respective strengths of confusion avoidance and clarity enhancement as systems. Avoidance offloads to the comma some of the semantic burden carried by words, achieving concision by compromising clarity.