Toilets, Sinks, Showers, Tubs and Disposals - Flat washer sizes ?

Could someone tell me what the "1/4" on a flat washer means?

Today I replaced the rubber washer on a Gerber bathtub valve stem. The old washer has "1/4" molded on it. I went to Ace Hardware store, they had packets of flat washers marked "1/4", "1/4M", "1/4L", with outside diameters varying from 9/16" to 19/32". The washer is 1/8" thick, and the diameter of the screw hole is 3/16". I just wonder why it's called "1/4", and what the difference is between "1/4", "1/4M", and "1/4L". I bought and used a 1/4L, but it's identical to the pack of 1/4 I already had at home. None of them fits as tightly as the original 1/4. :confused:

Thanks in advance for your help.

Here's a pretty complete answer.....
About Faucet Washers (http://faucet-fix.com/html/about_faucet_washers_.html)

Thank you very much. The difference between Trade Size and Actual O.D. is enough to confuse most people.

Note that the chart only shows the most common sizes.There is also "000",1/4M and above 1/2 is 1/2L,5/8.5/8L,3/4 etc all the way to 1(inch).All in both flat and beveled.

You don't find those large washers in much anymore but they turn up alot in older hose bibbs and shut off valves and the like.

Someone pointed me to this document, which shows the additional sizes that you mentioned.

http://korky.com/PDF/FaucetWashersandORingsCatalog.pdf

the size of washer needed is based on diameter of the seat surface. there are a great many faucet manufacturers and they dont have a standard size or style for seats. as far as i know