Soft tailoring is a popular term among bespoke aficionados but not very exact. If I should try to nail it, I would name two qualities: interlining and seams.
In a softly made jacket like the one above, you will find linen interlining but no horsehair or similar stiff material. You will also discover that the garment’s pieces are attached in flexible seams, most notably where shoulder and sleeve meet.
“Drape” in the outer chest and shoulder blades of the jacket is often associated with soft tailoring. However, we’ve seen many jackets combining strong built up shoulders and drape, especially in the 1930s and 1940s. Add to that many softly made form-fitting jackets and body coats in the 19th century with no drape whatsoever.
That said, soft tailoring and drape are related aesthetically and functionally. Both are about ease.
Photos: The Journal of Style