This is a question I often get so I'm going to answer it here "in a nutshell".
Coutil is a fabric that is engineered specifically for corset making. It can be a very fine herringbone weave but it can also be a brocade or a satin. What makes coutil coutil is ...a few things. Coutil has a high thread count, which means it's not inclined to stretch and bones are not likely to work holes through it. Coutil is not bulky, it's very fine and this coupled with it's strength make it the ultimate fabric for corset making and for the foundation of evening wear bodices. It is always made from natural fibers, either 100% cotton or a cotton viscose blend so it breathes and absorbs sweat.
Beautiful brocade coutil means you can make a corset from one layer of fabric and it will be both strong and beautiful. Use a layer of herringbone coutil cut on the crossgrain for the lining and a layer of satin coutil or brocade coutil as the exterior layer and you'll have a corset strong enough for tight-lacing.
Theaters have learned the value of coutil - a well made corset can last for years with a little care when made from coutil. But...not all coutil is created equal and there are companies selling "coutil" at what may seem amazing prices. Be wary of great priced coutil - high thread count means more threads required and more labour to weave so good coutil is costly. Some companies assume that because a fabric has a herringbone weave it is coutil, but that is not the case.
Coutil is not just the perfect fabric for corset making it's also used for period boned bodices, doublet foundations, basques to mount multiple layers of net to for a skirt, bust flatteners; anything that requires strong durable and fine fabric.
Farthingales has an extensive selection of coutil - over 20 qualities and you can order a sample pack.