My earlier discussion of heavy and light punctuation encompassed only today’s trivial differences in punctuation density, but the differences are much greater across the centuries. Samples from Ben Jonson illustrate the early 17th century’s predominant style of punctuation when writers punctuated based on prosody instead of syntax, marking wherever the reader should pause.
If you, my Sonne, should, now, preuaricate,
And, to your owne particular lusts, employ
So great, and catholique a blisse; Be sure,
A curse will follow, yea, and ouertake
Your subtle, and most secret wayes.
This earlier English literature shows exactly what’s wrong with the practice of punctuating whenever you hear a pause: by contemporary standards, you’ll over-punctuate.
The history of written English runs from heavier to lighter punctuation and from reliance on prosody to reliance on syntax. Logically, prosodic punctuation and heavy punctuation need not go together. To lighten punctuation, it's true you must omit punctuating some pauses, but in principle, punctuating for prosody allows degrees of punctuation density; presumably, you would punctuate the longer pauses and omit the shorter. Perhaps English didn’t follow that route because differences in pause length can be hard to ascertain reliably, but the reason for syntactic-punctuation’s lightness is clearer: an excess of syntactic punctuation confuses readers because syntactic elements are nested, whereas our means of punctuating allows only two levels within a sentence. Prosodic punctuation can be dense without confusion, since its only burden is telling the reader to pause.
The main reason punctuation is increasingly based on syntax is that writing is increasingly distinct from speech. How readers should render passages aloud matters less today; how readers should parse passages matters more. Although syntactic punctuation dominates, some writers disagree—and I don’t make any claims about the punctuation appropriate to fiction, dialog being peculiarly prosodic. Also, the purposes behind common punctuation practices conflict, with some accepted practices being based on prosody. The rule that a comma follows any introductory element is a prosodic rule, in contrast to a purely syntactic rule that would omit the comma after a restrictive modifier, such as an introductory “if” clause. Another example of contemporary prosodic punctuation is the use of a comma within a compound predicate where the verbs strongly contrast. A third example countenanced by some writers and grammarians uses commas for emphasis, a prosodic consideration that conflicts with syntactic rules under which commas set off nonessential, descriptive elements—usually amounting to de-emphasis.
Conflict between prosodic- and syntactic-punctuation practices sometimes confuses. The fundamental error of comma usage can be diagnosed as due partly to an appetite for prosodic punctuation: a reader often pauses before a coordinating conjunction. Another confusion leads to setting off restrictive adverbial clauses with commas. Still another prosodic temptation, which comes from the need to breathe when you read aloud, is to punctuate long passages. Temptations to separate a restrictive adverbial clause and to punctuate a long passage here reinforce comma-usage’s fundamental error of punctuating a compound sentence element. (HT: an old posting in WordReference.com.):
Maury licked his lips as Cherise, the dental assistant, leaned over him to adjust the table holding the sharp, shiny tools the oral surgeon would need, and wished his rotten old teeth were strong enough to pierce her lovely jugular.
The forum debated whether a comma goes after need. One commenter pointed out that it's ambiguous whether Maury or Cherise is the one wishing about Maury’s teeth, and the commenter suggested that a comma after need might clarify that it's Maury. A single extra comma doesn’t help, but a couple of commas—the other one after lips—would set off the adverbial clause beginning with as and ending with need. But we would be punctuating for prosody, using reading pauses to clarify meaning; from the syntactic standpoint, commas would improperly set off a restrictive clause. Creating prosodic breaks, such as interrupting sentence flow with a restrictive element, is the almost-exclusive function of the dash:
Maury licked his lips—as Cherise, the dental assistant, leaned over him to adjust the table holding the sharp, shiny tools the oral surgeon would need—and wished his rotten old teeth were strong enough to pierce her lovely jugular.
If, as there’s reason to expect, the trend toward punctuating for syntax instead of prosody continues, the future will falsify prophesies of the dash’s demise. As writing detaches from speech, prosodic punctuation doesn’t disappear, but writers can quarantine it within dashes.