Adjustable plastic ring sizer. A handy tool for the jewelry connoisseur.
 
To find your ring size:
Wrap the band around your chosen finger with the printed side facing out
Feed the end of the band into the mouth (like you would with a ziptie)
The sizer should sit where you would wear your ring
Once you have found a comfortable size, take note of the number the arrow is pointing to - that is your size!
 
Note
Your correct size should comfortably slide on and off your finger without becoming too tight passing around the knuckle. Shake your hand around, take it on and off a few times. Adjust until you find the sweet spot.