Colon (ko’-lon): Roughly equivalent to “clause” in English, except that the emphasis is on seeing this part of a sentence as needing completion, either with a second colon (or membrum) or with two others (forming a tricolon). When cola (or membra) are of equal length, they form isocolon.
Colon or membrum is also best understood in terms of differing speeds of style that depend upon the length of the elements of a sentence. The Ad Herennium author contrasts the slower speed of concatenated membra to the quicker speed of words joined together without conjunction (articulus).
I had a car. I had a house. I had a wife. Everything was great until my wife went nuts. She wrecked the car. She burned down the house. Then, she got a lawyer. Now, she’s out on bail. I’m living in an apartment and taking the bus to work. As far as I’m concerned things couldn’t get much worse, unless she finds out about my previous marriage. My previous wife disappeared in New Jersey without a trace. I was cleared of any wrongdoing, but try to get anybody in Jersey to believe it! They were all against me–unfair, unreasonable, uncharitable. I’ve been living here in Ohio for the past 12 years without a trace of wrongdoing. Did I say “Without a trace?” Whoops.
Definition courtesy of “Silva Rhetoricae” (rhetoric.byu.edu).