Styling
A Question of Scale
Layering cushions has less to do with how many you own than with how they relate to one another.
There is a point in a well-styled room when you stop noticing the cushions and start noticing the sofa. It reads as effortless, which is usually the sign that a series of decisions was made and then hidden.
Most of them were about size.
Build from the back
The back layer is rectangular. A rectangular cushion lies on its longest side and sits behind the square ones. This is not a preference, it is what the shape does: lying down it is shorter than the square in front, so it is never read above that square. It is read at the sides.
So a rectangle has to be meaningfully wider than whatever stands in front of it, or it disappears. Set a 16x24 behind a 16x16 and the two cancel out. Give it width to spare and it widens the group and gives it a horizon.
The tall rectangles are the exception. At 20 inches high, a 20x24 or a 20x28 rises above a 16x16 as well as reaching past it, so it can anchor an arrangement on its own.
Two squares need a real difference
Squares can sit in front of squares, but only when the difference is obvious at a glance. Around six inches, preferably more. An 18x18 in front of a 22x22 is a step, and reads as depth. An 18x18 in front of a 20x20 is a near miss, and looks like an accident.
Break the line, then close it
By now everything has a right angle and the eye wants relief. A round cushion supplies it, set forward of the square layer and slightly off center. Centered, it reads as a target and the group snaps into symmetry, which is the one thing a layered arrangement should never be.
A bolster is the last decision, not the first. It closes the composition, at the front or against the arm.
Odd numbers, and why
Group in threes and fives. An even number pairs off, the eye resolves it and moves on. An odd group has no obvious center, so the eye keeps traveling, and that traveling is the effect.
None of it needs a large collection. Three pieces at the right proportions do more than eight bought a pair at a time, because an arrangement is a composition, and compositions are built rather than accumulated.
The cushions
Start with the back layer
Rectangles first, then the squares that stand in front of them. Everything after that is adjustment.