Colon Use?

[PH]

From my previous post, I'm posting a revised question because of the rules concerning posting actual q's from the GMAT.

What's the general rule regarding Semicolon and Colons?

I know a semi is to be treated like any other punctuation, meaning it can be used interchangeably with a period.

What else about the Semi?

Also what about colons?

What must follow? and what must it be in agreement with? Is it supposed to agree with an object in the previous sentence? Or is it to modify something?

Please help, thank you.

[RonPurewal]

I know a semi is to be treated like any other punctuation, meaning it can be used interchangeably with a period.

yes, a semicolon is a lot like a period, and, from a purely grammatical standpoint, i suppose you could say that the two are 'interchangeable'. however, the rhetorical uses of the two are vastly different: you use a semicolon only when you want to connect two assertions that are intimately related in some way. (for instance, the second statement could offer a counterpoint to the first; the second statement could reinforce the first; they could be parallel statements; etc.)

semicolons are also occasionally used as substitutes for commas in lists, but only if the items in the list are really, really long or contain lots of commas themselves:
one nucleotide of DNA consists of a molecule of deoxyribose, a five-carbon sugar that forms half of the DNA's backbone; a phosphate group, which forms the other half of the backbone; and a nitrogenous base, which contains the actual genetic information that makes DNA so valuable.
i have never seen this particular construction on the gmat, and feel that it's somewhat unlikely to turn up, but you should know that it's out there (because it's the only instance i can think of in which the semicolon is not equivalent to a period).

--

the first part of this statement - 'treated like any other punctuation' - worries me a little bit, because it seems to imply that any other punctuation acts like a period. i'll assume that's not what you meant, and that those words essentially don't count. if they do count, then please re-post and explain what they're supposed to mean.

Also what about colons?

What must follow? and what must it be in agreement with? Is it supposed to agree with an object in the previous sentence? Or is it to modify something?

Please help, thank you.

2 primary usages for the colon:

1) to connect two independent clauses, where the second clause is an explanation of the first.
stevens creek boulevard in san jose is a culinary wonderland: a mere two-block walk leads past ethiopian, korean, japanese, irish, thai, italian, and spanish restaurants.

2) to follow an independent clause with a word, phrase, or list (not a clause) explaining, specifying, or clarifying the material of the original clause.
a complete weather station reading contains five elements: temperature, humidity, precipitation, wind speed, and wind direction.
in this usage, note that the part to be explained should be as close to the colon as possible. therefore, the above sentence is good because 'five elements' comes right before the comma, and the list of the five actual elements immediately follows the comma (so there's no 'break' in mental processing for the reader). if the independent clause had read 'five elements make up a weather station reading', that wouldn't be as good, because there's a big gap between 'five elements' and the list of the five elements.