Yes – same idea with different terms. I picked up the term “flatlining” around 2001 when I plunged heavily into historical sewing. Then it seemed others picked it up. I had to research (much later) what underlining was and realized it was the same idea but in a modern setting.

I, too, have made a modern jacket pattern that called for fusible interfacing on the full fashion fabric pieces. It’s indeed a modern (last half of 20th century) thing. But I agree it just makes the garment too stiff. I’d prefer now to mount the fashion fabric to non-fusible innerlinings specifically designed for tailoring. SO MUCH BETTER of a result! Glad your second version turned out well.

It seems that the historical community tends to call it flatlining, and contemporary sewists call it underlining. (I’ve just made a jacket, underlining the bodice pieces, the instructions said to interface all the bodice pieces using woven cotton fusible, which I did, but it came out a bit stiff, so second iteration I flatlined the pieces in a plain woven cotton. Same outer fabric, better result. “Speed tailoring” has a lot to answer for). My sewing is contemporary, but with some historical techniques.

Thanks for sharing, Val!