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How to Measure
Chest : Measure the thickest part of chest. (Usually behind the two legs).
Neck: The Girth of Necklace.
Length?: Make sure your dog is standing properly, not sitting, or laying down. Measure from along the dog's backbone from the base of the neck to 5 Cm front of base of tail. For sizing, you may find the following a helpful guide for a quick assessment ( inches refer roughly to length measurements). If in doubt, don't try the item on your pet if you would like to make a return:

SIZE CHART
SIZE	WEIGHT	CHEST	NECK/LENGTH / BREED
SMALL	5	13	8.25 / 8 YORKSHIRE TERRIER, CHH
MEDIUM	8	15.5	9.75/9.5 Yorkshire terrier, Maltese,MiniPin, Pomeranian
LARGE	11	17.5	11.25/ 11 Shih Tzu, Pekingese, Maltese, Poodle, Schnauzer, Pomeranian
X-LARGE	16	20.5	13/12.5 Shih Tzu, Pekingese, Schnauzer, Pug, Poodle
XX-LARGE	23	23.5	15/ 14.5 American cocker spaniel, Fox terrier
XXX-LARGE	30	26	16.5/ 16 English cocker spaniel, American cocker spaniel, Beagle