I’m assuming you want to build a bike starting for my frame-set (www.bertellibici.com/shop.php), but in case you own another brand or you find something interesting in a garage sale, remember before buying, to check the brand if i ist Italian, French, Japanese or British because those are the main four categories of every bike in the world. Every type has is own sizes and difference and sometime, it you are restoring a bike you might not be able to find the part they need .

The most common type is the British/ISO and is definitely easier to find components with that size espcially here in U.S. The difference between British/ISO and Japanese are only on the crown race (headset) whereas British and Italian have different thread (some says interchangeble but I never tried to prove it.)

There are lot of different type of frames but the most visible difference is:

Lugged frames with threaded head fork (Cinelli Supercorsa) or frame with visible welding frame-set with thread-less fork.

The only one I would use are the old-school lugged steel frame-set with threaded fork. Frame-set with visible welding can be a symptom of a cheap frame or at the opposite — really— a very high end quality. Usually frame made in China (not Taiwan) are made with visible welding and they are pretty recognizable. On the other side the welding can be symptom of a professional frame since the builder prefer not to use lugs because without them he can create a more precise geometry to fit the rider’s body.

Bottom line, since I’m not using my bikes for competition, I would rather use lugged frame because of their cleaner and classic design.