Updated 2/18/09 American & Efird (the thread company) has a much more user friendly six page pdf seam class chart using the older military designation which is the one that most people still use. The first page is an index; subsequent pages show diagrams with the needle path shown in red. This pdf is standard reference material to keep on hand when filling out forms and instructions to contractors. The ASTM system mentioned below organizes seams entirely different with numerical designations and is not used quite as much.
I keep forgetting to mention that I have all of the seam class specifications and pictographs in pdf file format available for download on my site at Designer-Entrepreneurs. These specs were originally developed in accordance with standards found acceptable by the US military and were adopted for use by military contractors over 50 years ago. In fact, one would think it’s time for everyone else to use them too. These are the standards formerly known as designation 751.a and cost over $50 from the GPO but now, here you can get them free. Even if you’re not one of those twisted individuals who enjoy reading and collecting specs -like yours truly- these will be useful in communicating desired seam details to your sewing contractor, eliminating any ambiguity regarding required product quality standards. To whit, specifying these standards in a contract make your product quality specs iron-clad so be nice and print the pictographs of the specific standards you want and provide them to your contractor because it’s unlikely they have them handy.