Of the difference between zeugma and syllepsis

I have always understood the difference between zeugma and syllepsis to be that syllepsis is used to create a semantic dissonance with intentionally humorous effect. For example, here is Ambrose Bierce's definition of the word piano from his The Devil's Dictionary

piano n. A parlor utensil for subduing the impenitent visitor. It is operated by depressing the keys of the machine and the spirits of the audience.

Zeugma, on the other hand is supposed to be used for dramatic or felicitous effect. Or so I was told when I studied rhetoric.

The sad fact is, the meanings of these two terms have become inextricably yoked together (ha ha) over the years, blurring the distinctions and our ability to articulate them (again, ha ha).

A syllepsis is a zeugma that's wrong.

For example, "Friends, Romans, countrymen, lend me your ears" is a zeugma. Each of the first three words is a vocative for the rest of the sentence.

"You held your breath and the door for me" is a zeugma that is a syllepsis, because "held" is used to mean two, incompatible, things. It's hard to imagine a syllepsis that wouldn't sound jarring and incongruous.

Based on the definitions in my first "real" dictionary (New World Dictionary of the American Language, Second College Edition, 1980), I've always understood zeugma to be the clever one and syllepsis to be the boring one.

zeugma: a figure of speech in which a single word, usually a verb or adjective, is syntactically related to two or more words, with only one of which it seems logically connected (Ex: The room was not light, but his fingers were)

vs

syllepsis: a grammatical construction in which a single word is used to modify or govern syntactically two or more words in the same sentence, though it can grammatically agree with only one of them (Ex.: either they or I am wrong)

Actually, I find I have a cavil with the above definition of zeugma: although it contrasts "logically connected" with an implied "illogically connected," the example illustrates "concrete" vs. "figurative."

Anyway, the point is that, according to these two definitions, syllepsis represents a grammatical strain; zeugma is more of a semantical mixture.

Now, from another "real" dictionary (The Compact Edition of the Oxford English Dictionary, Complete Text Reproduced Micrographically, 1971):

zeugma: A figure by which a single word is made to refer to two or more words in the sentence; esp. when properly applying in sense to only one of them, or applying to them in different senses; but formerly more widely, including, e.g., the use of the same predicate, without repetition, with two or more subjects; also sometimes applied to cases of irregular construction, in which the single word agrees grammatically with only one of the other words to which it refers (more properly called Syllepsis).

vs

syllepsis: A figure by which a word, or a particular form or inflexion of a word, is made to refer to two or more other words in the same sentence, while properly applying to or agreeing with only one of them (e.g. a masc. adj. qualifying two sbs., masc. and fem.; a sing. verb serving as predicate to two subjects, sing. and pl.), or applying to them in different senses (e.g., literal and metaphorical).

Here, the overlap between the two words is brought out a bit more; however, the distinction can still be seen: zeugma causes a word to serve double (or more) duty in terms of sense; syllepsis does so in terms of syntax.

A practical if arbitrary distinction: syllepsis should denote a grammatical incident, for example a disagreement as "None of these texts are known in their original form." [the verb agreeing with "these texts"] instead of "None of these texts is known in its original form." [the verb agreeing with "none"]

The ambiguity could be prevented by the restricted use of "zeugma" as a figure of speech only.

Breaking the "covered" or "connected" words apart and connecting them with the "unifying" word makes clear the difference. Ex: "Friends, Romans, countrymen lend me your ears." This breaks apart to "Friends, lend me your ears; Romans, lend me your ears; countrymen, lend me your ears." "Lend" fits with each of the linked words correctly and with the same meaning of "lend." Ex: "Piano: played by depressing the keys and the spirits of the listeners." This breaks apart to "Piano, played by depressing the keys; also, depressing the spirits of the listeners." "Depressing" has a sense in each of these phrases, one being physical, the other being psychological. It is this unexpected change of "matrix" that induces humor or dis-settlement in the reader. I leave it to the reader to decide which is which. [Hint: they are in alphabetical order.]