The Carhartt 874 — confusing on the model number, since 874 is also a Dickies pant — is the duck-canvas double-front workhorse Carhartt has been making in some form for forty years. The FLEX version came out about ten years ago and added stretch into the canvas weave. They look identical on the rack. They feel different the minute you squat.
Original 874 is straight cotton duck. No stretch. Stiff out of the bag, breaks in over about two weeks of wear. Ends up softer than denim, harder than chinos, and lasts longer than both. It's a pant for guys who like a pant to feel like a pant — substantial, structured, opaque.
FLEX 874 is the same duck canvas with a small percentage of spandex worked into the weave. The pant moves. You squat, you kneel, you reach up to a top shelf — the pant gives. Same fabric weight, same look, just engineered to stretch.
Trades where the original wins. Welders. Anyone working with sparks or molten metal — spandex melts and the original is pure cotton, which chars but doesn't melt to skin. Oil-and-gas guys with FR requirements. Anyone in heavy demo where the pant is going to get torn anyway and you want the cheapest replacement cost. Old-school carpenters who just don't want a pant to stretch on principle.
Trades where the FLEX wins. HVAC, where you're crawling into closets. Plumbing, where you're under sinks. Electricians who go from ladder to crawl space and back. Anyone who squats more than twenty times a day. Drivers who are in and out of the truck constantly.
The FLEX runs about $15 more. Worth every dollar of it for the trades that need it. Money badly spent for the trades that don't.
One more practical note: the FLEX wears a half-size more snug than the original. The stretch fabric pulls in at rest. If you're a 34 in the original, you might want to try a 34 in the FLEX first and see — most guys end up in the same number, but a few find the 36 fits the way the 34 used to.