When a dog's tail wags, it means they are friendly.

Question

There are some websites saying that a dog's tail wagging means they are friendly. However, there are other people that say tail wagging could mean a whole range of emotions.

Answer 1

There are a variety of reasons for a dog to wag their tail. An article by petMD explains the multiple reasons. The article states, "When a dog is expressing happiness or friendliness, their tail is often neutral or slightly upright position and wagging at a moderate speed". However, the article also mentions that wagging can mean fearfulness, curiosity, relaxation, avoidance, and aggression. So, a dog wagging their tail does not only mean that they are friendly.

https://www.petmd.com/dog/behavior/why-do-dogs-wag-their-tails

Answer 2

Seeing a dog wagging their tail and immediately assuming they are friendly is the wrong assumption. Tail wagging is a way for dogs to single their emotions, and while frequently these are happy emotions, this is not always the case. While it does represents happiness in a dog, Door County Pulse says the technical meaning is " a dog is mentally aroused and engaged with what is going on in his environment." 

As this is a sign of emotions, not always positive, people still must be cautious around dogs wagging their tails. Dogs will still defend when their tail is wagging, and can still bite someone. 

https://doorcountypulse.com/pet-talk-a/

Answer 3

Tail movements from dogs indicate a wide range of emotions. However, according to the American Kennel Club, tail wagging can serve "the same communication functions as a human smile, a polite greeting, or a nod of recognition". Tail wagging is the canine equivalent to facial expressions in humans and thus, the emotions and demeanor maintained and projected by dogs are subject to change.

https://www.akc.org/expert-advice/lifestyle/why-do-dogs-wag-their-tails/