The 50-50-90 rule: Anytime you have a 50-50 chance of getting something right, there's a 90% probability you'll get it wrong.
Q:

Dear 100 Hour Board, When And Why did Wedding Rings Go on the left Hand?- Anxious To Get Married

A: Dear Anxious,

See Board Question #24376 for a brief history of wedding rings (along with a handy link to a not so brief history), brought to you by de novo.

Board Question #23584 also discusses the subject.

This Wikipedia article also includes some traditions and random trivia regarding wedding rings, and which fingers/hands they are worn on.

Cheers,

-not anxious to get married