Two spaces vs. one space?

[AOII Angel]

I read this yesterday and was so happy to see it! I have to correct my reports everyday, and the question of one or two spaces has plagued me for a long time (though not enough to look it up!) I learned to type on an old manual typewriter (with no letters on the keys) that my middle school found somewhere and then on electric typewriters in "keyboarding" class in HS. It was drilled into our heads to space twice after the period at the end of a sentence but only once after a period for an abbreviation, etc. I never knew the rules changed with computer fonts. Nice to know! I'm OCD like that so this rule will go into immediate practice.