Square throw pillows are the most common shape. They typically come in 16-, 18- or 20-inch versions, but can be found in larger or smaller sizes. If you like to change up the look of your décor frequently, stick to square throw pillows in standard sizes, which will make it easiest to find replacement pillow covers.
Round pillows are much less common than square versions, but can be a beautiful choice when used to balance the straight lines of square and rectangular pillows.
Cylinder-shaped bolster pillows get their name from their intended use: They’re meant to prop up your arms or back when seated, and are most commonly used on beds.
Lumbar throw pillows have an oblong, rectangular shape. Like bolster pillows, they were originally designed to provide back support, but are now used more often for ornamental purposes, usually to offset square pillows or decorate an armchair.