Down pillows for side sleepers need to fill the area between the crown of the shoulder and the side of the head. We like to call this area the ”Comfort Gap”. Side sleepers typically need either firm pillows or extra firm pillows. The “Comfort Gap” distance can change depending on the width of your shoulders and firmness of your bed. The more broad your shoulders the larger the ‘Comfort Gap’. The softer your mattress the smaller the “Comfort Gap.”
Pillows for stomach sleepers need to be soft. However, where you place your pillow will impact what density pillow is right for you. Many stomach sleepers sleep on top of their pillow, with some portion of their chest compressing the pillow, treating it almost like a body pillow. In this case a medium pillow may be perfect. However, if you primarily place only your head on the pillow then you need to look for a soft density pillow so that you don’t strain your neck muscles.
With pillows for back sleepers, how you position your pillow while on your back can significantly impact which density pillow you should choose. If you sleep with your pillow directly under your neck and the top of your shoulders then you need a firmer pillow. But if you sleep with the pillow more towards the top of your head then you need a softer pillow to prevent your chin from being pushed into your chest.