Foundation pillows on a sofa or sectional are typically larger in size and support the accent pillow which is equal in size or slightly smaller. A good rule of thumb for accent pillows is to use them to draw in the pops of color while your main seating piece stays fairly neutral. Sizing is a preference, but over the years pillows have played more of a role in home decor as they are larger in size and bolder in design. 

Foundation or compliment pillows are usually 24" or so in size. With corner placement, they help dictate and define the look of the space, creating a feel for the room. The depth of the sofa may determine the style of pillow you use while the length of the sofa may determine the amount of pillows used. For instance, a deeper sofa that works for your partner who is tall, may require a nice supportive feather pillow for the shorter household members to have more support. If the sofa is 108" long, you may want to fill each corner with pillows all the way to the center or add a pair on each side. If you have a studio sofa that is 60" - it's probably best to limit the pillows to two or three and adjust the size.

On a larger seating piece, the accents pillows are the next size down, typically ranging from 22" to 20" and this pillow makes a nice print. It's good to keep the colors harmonious and mix it up with contrast welt and various designs. As you move inward you can drop a size or leave it open. On a sectional thats a U or L shape, the pillow array placed in the corners can be repeated in the center. A nice lumbar in the center or a round accent pillow can add a fun pop of color or design element. Changing out pillows is a great way to create a whole new look in a space.

Foundation or compliment pillows are used in the corners and start the color way. It's nice to have these be a solid pop of the primary accent color, have mimimal pattern and maybe add a small detail like contrast welt. These pillow covers can be made from a different fabric, but in the same color, to add a bit of texture and depth. The accent pillows are used next in the pillow scheme and can be a fun print like vintage florals, tie dye, stripes, nautical themes, embellishments, coastal themes and Ikats. 

Pillows work their way from the outsides in and for a sectiional you can pile them on with wild abandon or line them up all the way to the center. In an accent chair, it's nice to have a lumbar pillow for lower back support as well as spreading color around the room.