Every pet has different claws although most are similar in size and shape in a particular pet. The first time you use the Zen Clipper, you will find that the red part of the claw that ensures blood supply (the wick) does not go into the cutting area of the blade. The patented conical blade ensures that, no matter if you are a professional or a beginner, these are not damaged (it's important to use the correct size). Sizing guide: - XXS - 1.5 mm hole - for smaller birds, kittens, ferrets, small reptiles & animals such as guinea pigs - XS - 2 mm hole - for most adult cats - S - 2.5 mm hole - for larger cats, larger birds, puppies, large reptiles and very small dogs - M - 3 mm hole - for small dogs and larger puppies - L - 3.5 mm hole - for dogs from around 9 to 18 kg | 20 lbs to 40 lbs - XL - 4 mm hole - for dogs from around 11 to 23 kg | 25 lbs to 50 lbs - XXL - 5 mm hole - for medium to larger dogs, approx. 23 to 30 kg | 25 lbs to 60 lbs