OK -- I know that the numbers on either end of a runway refer to a bearing (with the trailing zero dropped off) and that they always differ by 18 ('cuz they're 180 degrees apart). What I've always found confusing:

Does the number refer to the where you'd be heading or where you'd be coming from?

For example, if an a/c is coming in from due East, he's coming from bearing 90 degrees, and he's heading toward bearing 270 degrees. If he flies straight in, does he land on runway 9 or runway 27?

Likewise, if he takes off directly in to West, does he taxi to runway 27 (where he's going) or runway 9 (where he's coming from)?

After all these years, you'd think I ought to know this stuff, but ...

The runway number indicates the orientation of the runway with respect to the direction of takeoff. Therefore, landing from due east you would land on 27.

And the numbers are the approximate heading. For example, in DTW 21L, 21R, 22L, and 22R are all on a heading of 215.5 degrees. 22L used to be 21R when there were only three parallel runways and was renumbered when 22R opened. I guess 21R, 21RC, 21LC and 21L would have been confusing.

Someday they might think I'm a tewwowist if they see the "secret" info available on the net in my hand on a plane, but I like to print out an airport diagram and look out the window at the taxiway numbers to see "where I am" as we taxi out and in. I know what gate I was supposed to arrive at, but often I see a gate change before they announce it and if I have a short connect time, get ready to run.

For example, go to http://www.airnav.com/airport/KATL and on the lower right side get the .pdf of the Airport Diagram.

OMG, FIVE parallel runways! How to number them?

Years past the zero magnetic deviation line ran right thru Georgia/Florida, so numbers were actual. See the upper right on the diagram, showing 4 degrees west variation. The runways were originally built east west, ie 90 degrees and 270, for runways 9 and 27. (assuming land is available and mountains or prohibited areas do not interfere, point the runway to line up with the most common prevailaing wind.)

Note on each runway there is the actual current heading, all five are 94 and 274 degrees. Darn wandering magnetic pole. The headings are magnetic, so as the pole moves, every now and then a runway gets renumbered to be closer to what the planes compass reads. Helps the pilot line up. ^ A good thing. The mag pole moves, the runways point the same direction. Unless you live in earthquake country.

Since you cannot have five runways numbered 9, when there are parallel runways they get numbered Left and Right, so 9L and 27R are the same patch of concrete. Most airports do not have enough land for more than 2 parallels in any one direction, but ATL has five.

So after you use up 9L and 9R, you go to the closest and the next pair is 8L and 8R. The new southern fifth runway that just opened last year? Runway 10.

And of course, as the pole moves, so the runways might need renamed. One of the airports I used to work at went from 04 to 05.

They just changed 27L at ORD because of the expansion project going on.. so now it is 26.