The whole discussion came about because of a simple question , ""How does one know if it is a TRADE PAPERBACK or a MASS MARKET PAPERBACK? Anyone who has been in publishing or book dealing for any length of time does not even have to think about this, but I bow to the purity of such a question asked in all sincerity.
Soft-cover vs paperback? Unless books are bound in plush or someone's old pillow, they are not soft. Paperback is hardly pejorative - but rather descriptive. The bibliographical word used is wraps - or wrapper (as far as I am concerned wraps is a misnomer as goes physically descriptive aptness - I have a whole diatribe about this very subject with which I shall not bore you)
But consider that, leaving aside any considerations of ideas and the such printed marks found within, consider that paper is the very heart of a book, so why should a book not wear its heart on a sleeve, so to speak? In fact paper covers (wrappers) are nothing new - they have been around at least since the eighteenth century.
MASS MARKET PAPERBACKS (Think: revolving racks, etc.; think: fit in your pocket - hence the famous Pocket Book publisher; think: bright, often garish covers - think: what does this thinly clad woman on the cover have to do with Steinbeck?; think: collectability ... Much of paperback collecting is centered around cover artists such as Bergey, Meltzoff, Powers and Avati and such sub-genres as Good Girl Art and Map-Backs, but a goodly amount of collecting still focuses on authors):
Golden Age Size (approximately 4 3/4" x 6 3/8" ... used through the 1940s and 1950s. This is where much of the collecting occurs.)
Thereafter - Size is approximately 4 1/4" x 7" (recently some publishers have been experimenting with a taller mass market size)
TRADE PAPERBACKS (Think: Digest-sized; think "This does not really fit in my pocket very comfortably"; think: college courses and "serious" subjects, although any subject, including fictional are game; think: often stodgy cover decoration - this last has changed significantly through the years; think: low collectability - trade paperbacks have yet to establish themselves as a collectable field - there are exceptions within the field, which collecting activity is series-driven or author-driven)
Sizes vary ... approximately 5 " x 8" or 5 1/2" x 8 1/2" + -
Harper Torchbooks, for example, was one of the most commonly recognizably trade paperback series on the market. There are many others. Many, many publishers issued trade paperbacks.
In fact, one possible way to view the difference between mass-market paperbacks and trade paperbacks was that completely separate publishing concerns sprang up to issue mass market paperbacks .. ACE, Pocket Books, Penguin, Lion, Lancer, Dell, Pyramid, etc., etc., etc.; while trade paperbacks were generally issued by already established trade publishers (publishing to/for the trade). Of course there were exceptions ... the history of publishing is a record of concerns established on constantly shifting sands.
We've seen what had been solely mass-market publishers enter the realm of hardcover publishing.
Then there are the OVER-SIZE PAPERBACKS - the beached giant squids of the paperback world.
8 1/2" x 11" .... and larger. Almost to a one these Awkward Things are an evolutionary mistake.