The similarity required of the viols in a chest of viols usually meant that they were made by the same maker, and sometimes were ordered in sets. Similar viols were desirable because they would blend better, and also stay in tune with each other better than more disparate instruments.
In a group of viols, similarity was typically indicated that they were made by the same craftsman or purchased together. These similar viols were preferred for their improved harmony and better tuning compared to less uniform instruments.