Three inches of polyurethane foam, sandwiched between two 5" layers of cotton batting (that adds up to 13", doesn't it?  Actually, by the time it's pressed into the outer cover, it's 10+")

A triple-decker sandwich - two layers of 2" foam, with 2-1/2" of cotton batting between and 2-1/2" on both sides; provides a firmer support

We use a 252-coil innerspring, using 13-Gauge bed coils, with insulator pads on either side to protect the next layer, 3/4" high-density foam (each side).  This is followed by 2-1/2" of cotton batting per side.  It retains its comfort and its tailored look over long periods of time.