Snout-vent length

Snout–vent length (SVL) is the length of an animal's body. It is measured from the tip of its snout to its rear end. The snout is the part of the face at the front. The tail (if any) is not counted, nor is any protruding teeth or tusks.
So, in a frog, it is the length of the frog's body but not the frog's legs. For turtles, carapace length (CL, the length of the shell) and plastral length (PL) are used.
The SVL can change depending on whether the animal is alive, dead, moving, or still. It can change if scientists have used chemicals to preserve its dead body. Scientists also use snout-vent length on fossils. With fossils, they use osteological correlate, which is a way the bone can match the rest of the body. Precaudal length is one osteological correlate. A scientist can look at the animal's SVL, weight, and body and may be able to tell if it is male or female or how old it is.
Advantages.
Scientists use snout-vent length instead of the animal's whole body length because it does not change much. In some animals, young ones do not have tails or do not have large tails.
Methods.
The scientist or other person can measure snout-vent length with dial calipers or digital calipers.
The scientist can use other tools to hold the animal still. For example, snake tubes, "Mander Mashers," or a "Salamander Stick."
Limitations.
The term is useful for mammals. It's not so useful for animals where the tail is an important part of the body. For example, fish have tails that are parts of their bodies. But horses and elephants use their tails to hit flies but not much else. Dinosaurs that walk on two legs are a good example too. It took a long time for dinosaur scientists to understand this. Snake tails go far past their vent.