A relation R on words is a similarity relation if it is reflexive, symmetric and it is generated by letters. A word w is said to be R-square–free, if it has no factors of the form uv where u and v are related by R. We give a new proof for the result of Kärki [Fund. Inform., 116 (2012)] that the smallest similarity relation that gives an infinite square–free word is generated by six pairs.We also characterize all trees of order six corresponding to the similarity relations that admit a square–free word.