This is the perfect pillow formula for your posture, according to a physical therapist

Your relationship with your pillows is just about the closest in your life. After all, you do fall asleep and wake up next to them every day. And finding your perfect feather-stuffed partner might just be even more important than finding your romantic one.

The way you sleep has a major impact on your spine—and ultimately, your posture—and your pillow can have a major impact. “If you’re sleeping with a pillow that is the wrong size for you, your neck won’t be able to sit in a neutral position, which means your head is going to rest too far in one direction toward either shoulder,” says Jeff Brannigan, director of programming at STRETCH*D. “Over time this will create misalignment in the spine and negatively impact your posture.”

Because of this, you’re going to want to put together the right pillow formula for your go-to sleep position. If you sleep on your back (which is best for your posture and alignment), you’ll want to choose a firm pillow that supports your neck. “For maximum support, you need a material that gives way to pressure and stays put,” says Burke Morley, vice president of brand at Purple, a mattress and pillow company. This means that it should hold firmly under your neck, but give a little bit beneath the weight of your head.

If you’re a side sleeper, Brannigan suggests picking a pillow that allows your neck to sit in a neutral position, which will keep your spine neutral (when all three areas of your spine—cervical, thoracic, and lumbar—are properly aligned) throughout the night. Ideally, you’ll also hold a small pillow between your knees for added support throughout your body

And if you’re a stomach sleeper (which, I’m sorry to say is “the worst position to sleep in,” according to Brannigan), you can opt for a softer pillow, but consider changing things up. In any case, grab a pillow and assume the (back or side) position, and the two of you—and your improved posture—can live happily ever after.

Using the right pillow isn’t the only thing you can do for your posture while you sleep: Here’s why your mattress matters. Plus, the best stretches to do before bed.