My cutter at Henry Poole sent along this photograph of my pattern the other day. It is a first cut at the parts of a new jacket based on measurements taken in San Francisco last Fall.
Patterns are usually made of a sturdy substance called oaktag. They need to be sturdy so they can be laid on the cloth of the garment in process and weighted down rather than pinned (pinning tends to disrupt the lay of the cloth, making it difficult to cut the two layers identically). Then the pieces of cloth - and a three piece suit has nearly twenty of them - can be marked with tailor's chalk before being cut out with shears.
This particular pattern will be turned into a basted jacket for a first fitting in February. Any necessary modifications will be marked on the jacket at that time and later made on the pattern so that it is the repository of the cutter's understanding of how to make a garment that fits the client.
Fit is after all probably the principal single justification for bespoke tailoring. And, since it often takes as many as three jackets to fine tune a pattern so the fit is perfect, one can argue that patterns like this one are, along with people, the principal assets of a tailoring firm.