Description Rex Specs Ear Pro by Zeteo Tech was developed specifically to protect military working dogs from temporary and permanent hearing loss that can arise from noise exposure. Whether on a mission, at the range, on a motorcycle, or at an AC/DC concert — Rex Specs Ear Pro will protect your dog's ears and can limit your dog’s hearing damage over time. 1. Head Circumference Measure the circumference of your dog’s head at it’s largest point (just in front of the ears and under the chin). If you are between sizes, please go up in size. Head Size 13” - 15” 2 15” - 17.5” 3 17.5” - 21” 4 I got this for my dog with the intention of using it for fireworks in July. His head measures 15” and I ordered the size 2, but I feel like it’s too snug on him. I tried it on him briefly, but did not end up using it. Hopefully some other lucky pup can use it!