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Sea to Summit says:
Aeros™ Pillow Cases go over the top of our Aeros™ Pillows for easy washing or for use with spare clothes or a down jacket as a minimalist pillow alternative.
While all our pillows can be machine washed with the valve closed, some of us still prefer a more traditional solution. The Aeros™ Pillow Case can be slipped over our Aeros™ Pillows through the 20cm hidden zip, then quickly and easily removed for washing.
To prevent slippage during the night, the Aeros™ Pillow Case’s base fabric is compatible with the hook-and-loop fastener patches that will come standard with all Sea to Summit Sleeping Mats from mid-2019. Current Aeros™ Pillows can be placed inside the Aeros™ Pillow Case for seamless integration with this new feature.