This is the decorative pillow range specifically, as distinct from the functional headrest pillows that strap to a chaise. These are the pieces you add to a sofa, sectional or deep seating group for back support and colour.
Square throw pillows run from 12 to 22 inches. Smaller sizes work as accents on a club chair, larger sizes provide genuine lumbar support on a deep sofa. Mixing two or three sizes across a sofa reads far better than lining up identical squares, and odd numbers look more natural than even ones.
Bolsters come in rectangular and round shapes. The rectangular version gives lumbar support at the small of the back, which is what makes a deep seat comfortable for someone shorter. Round bolsters finish the ends of a sectional run and add a more tailored, considered look.
These use fade resistant outdoor fabric with fill that drains and dries rather than holding water and souring. That is the practical difference between a pillow you can leave on the deck and one that has to come inside every night.
For headrest pillows that attach to a specific chaise or chair frame, and for the full pillow range, see Outdoor Pillows.