confusion on "anti-symmetric" and "symmetric" Hi guys, I am a physics sophomore at next term, recently I am doing a reading on Naive Set Theory on my own. However, I got a few confusion. The books said that if A is a subset of B and B is a subset of A, then A=B, but this set inclusion is anti-symmetric, on the other hand, based on the axiom of extension. Two sets are equal iff they have the same elements, then. if A=B, then it is symmetric. My question is: a.) What meant by being symmetric & anti-symmetric b.) what is the difference between the two approaches to "A=B"? Thank you so much for reading :D have a good day.