First Fixed project!

You need to decide whether you will be building your parts around your frame or buying a frame that will accommodate the parts that you want/have. Frames have different standards and if you are getting an old frame you might have some difficulties getting some parts for it. Things to look out for :

1. http://en.wikipedia.org/wiki/Bottom_bracketshell width, most common are : 68mm and 70mm
   2. Rear dropouts spacing will decide what hub you will be using
   
2. Headtube of the frame will indicate the size and type of the fork you will need to use.
   
3. Not very important but also to consider seatpost tube diameter (most popular 27.2mm)
   
   

Just make sure that the frame you are getting wont cause you a lot of trouble finding parts for it. (Some old frames have outdated standards)