All of the above are correct, some additional details. 700c where mainly sew-ups while 27 where mainly clinchers. in the late 70's early 80's, when nice quality bike came from Japan, there was a revolution in tires. Good quality clinchers where being made in both sizes, by good quality I mean kevlar bead light weight tires. So you could hav a folding tire, for a spare, that you could pump up to 100+ lbs, with the convience of a clicher, ie no glue to change a tire. And the race was on, Bicycling magazine had an article, titled something like 'Which will win; 27 or 700'. Need less to say we now know who won(and which of us backed the wrong horse). The bottom fell out of the Japanese bike import industry due to increased tariffs, and with it went any support for the 27" wheel. Along with some good components not made by Shimano, my Suntour Cyclone will still stand up to campy, and that is a loss for us all. If you want to get some blank stares, and cause mass confusion, go in to your LBS and ask for 27x1 tires with a kevlar bead.