Each VitalSleep case has a size printed on the bottom. The recommended size for  men will have the word Regular on the protective travel case . 

The recommended size for women will have the word Small on the bottom of the protective travel case.

There is also a small letter R imprinted on the right inner backside of the regular mouthpiece and a small letter S imprinted on the right inner backside of the small mouthpiece. 

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