This, together with balance, is what counts, rather than weight. I once forgot my strap and had to borrow a piece of string to bodge something at the gig. After a couple of hours, it felt as if someone was sawing into my shoulder, even with a light (8 lb approx.) well balanced instrument. I eased it a bit by putting apiece of thick cardboard torn from a box between the string and my shoulder, but I was sore for a couple of days afterwards.