Part of the ‘Antique Fabrics‘ series, this fabric isn’t necessarily extinct, but it was more popular in the past than the present.
Serge is a soft, durable twill-weave fabric. The pattern of the weave is very much like that of denim – steeply diagonal and reversible.
In the 19th century, serge was a popular choice for military uniforms, especially when made with worsted wool. It also saw use in suits and coats. It’s still around today, but not as ubiquitous as it once was. Silk serge may be used for lining.
In modern usage, serge may also refer to a high-quality wool suiting – just to keep things confusing for you.
If you’re looking for a good-enough alternative, consider a gabardine fabric.