Butterfly-shaped pillows have recesses at the sides, and they solve a geometry problem specific to side sleeping.
The Shoulder Problem
Lying on your side, the shoulder is the widest part of your body and it sits directly under the pillow’s edge.
On a flat pillow the shoulder gets compressed into the mattress while the head is held up. The neck then has to bridge between a shoulder pushed down and a head propped up, and it does that by bending sideways for the whole night.
That is where one-sided neck and shoulder stiffness comes from.
What the Cutout Does
- Gives the shoulder somewhere to sit rather than under the foam
- Lets the pillow support the neck at its own height
- Removes the shoulder from the height calculation entirely
- Reduces pressure on the shoulder you are lying on
- Stops the pillow being pushed away by the shoulder
Descriptions that put it this way, that the cutouts take the shoulder out of the equation, of the kind at bespillow.net, are describing the mechanism accurately rather than selling a shape.
Using It Properly
The shoulder goes inside the cutout rather than against the edge of the pillow.
People frequently place a butterfly pillow like an ordinary one and then find it too narrow. Sliding the shoulder into the recess is the whole point, and it needs doing deliberately at first.
The Wing Rests
Some butterfly designs raise the outer sections so an arm can rest on them.
Side sleepers usually have one arm underneath and one in front, and both end up somewhere improvised. A raised wing replaces the second pillow people otherwise use for this.
The arm underneath is a separate matter, and tucking it under the pillow usually means the pillow is too low.
Broad Shoulders Need More
The cutout helps and does not eliminate the need for adequate height.
Anyone with broad shoulders on a firm mattress has a large gap to fill, and a pillow that is simply too low will not work whatever shape it is.
Back Sleepers Do Not Need It
The cutouts are irrelevant on your back, which is why the plain wave profile exists.
They Are Wider
Butterfly pillows are broader than standard, which is worth checking against your pillowcases.
Change Sides
Sleeping on the same side every night loads one shoulder repeatedly.