desktopsallye (see Links) is described as “An Entirely Semi-colon Free Website”, but as Sally and I both know, I use it fairly often in poems. The Chicago Manual of Style sits on my reference shelves; I check it rather than Lynne Truss whenever I need an authoritative answer to a stylistic question. Of the semi-colon it says, “…it is still occasionally useful to mark a more important break in sentence flow than that marked by a comma. It should always be used between two parts of a compound sentence when they are not connected by a conjunction.” Chicago gives a number of other indications where the semi- should be used, but for my poetry that first one is enough: it separates the the two parts of a compound sentence. I write a lot of compound sentences.
“The comma indicates the smallest interruption in continuity of thought or sentence structure.” (op. cit.)
The strongest stop is the full stop.
The colon is an emphatic stop, short of a full stop, used to mark a break in grammatical construction, where two clauses are linked which might otherwise form single sentences, or where a second clause contains an illustration or amplification of the first.
Chicago is also good on other punctuation marks, including dashes, braces (parentheses) virgules (slash) etc.
For many years, on the wall of my office, before I attained what Goldsmith calls “O blest retirement, friend to life’s decline…” I had pinned up the following quotation from John Donne’s Meditations. I love it still, because I share his sentiments entirely and very deeply; because I admire the rippling flow of words in what is a single sentence (check it), and – perhaps more quirkily – because the original orthography appeals to me.
No man is an Iland, intire of it self; every man is a peece of the Continent, a part of the main; if a clod bee washed away by the Sea, Europe is the less, as well as if a promontory were, as well as if a manor of thy friends or of thine own were; any man’s death diminishes mee, because I am involved in Mankind; And therefor never send to know for whom the bell tolls; it tolls for thee.
John Donne: Meditation 17, 1624