A nasal pillow seals against the edge of each nostril rather than around the entire nose. The pillows open into the nostril but are not inserted inside to seal. Nasal pillows are helpful for people who sleep on their side or stomach, or for those with facial hair. There are different pillow sizes available for sale for ease of fit.
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Though not for everyone, in my clinical & personal experience nasal pillows have proven easier to use in maintaining seal during/after position changes. Nasal pillows offer less linear leak opportunity than nasal masks, which themselves offer less linear leak opportunity than full face masks.