The phrase "at the back of" implies a position or location that is behind or beyond a certain point. Antonyms for this phrase could include "at the front of," "in front of," or "before." These words denote a position that is ahead of or in front of something else. Other antonyms could include "in the middle of," "within," or "inside," which describe a location that is situated within or surrounded by a certain space. Conversely, antonyms for "at the back of" could also include descriptors that indicate a location that is on the side, such as "beside," "next to," or "adjacent to.