Akiyama Kyusaku.....About The Periods of Tsuba.
"We can see the difference of taste according to periods. Thinking about the question that may crop up amongst amateurs in the future, someday we must study them in order of periods, then it will be very difficult to make a satisfactory classification in chronological order from Genpei, to Hojo, to Akasaka, etc. As with lacquer and other things where it is much easier, because there were many wars, and tsuba were, of course, sharing the fate of swords in battles, and very few records and traditions are in existence. So that the best way to study them, is by those that are left to us, without following tradition and unreliable stories.
As a method of judging let us find five or six kinds of very old make, as best examples, by which to study the others, it is the shortest method. Of course the period covers 500-600 years, and during that space of time, there were several 10,000 makers, and the number of tsuba is several hundred thousand pieces, so that it is needless to study poor pieces made by common makers."

Annotation by Robert E. Haynes.....
"It would seem that Akiyama was the first student of fittings to think about studying them in chronological order. Before his time such methods were not even considered. Only lists of the known artists were put into the early books, and these did not mention the dates, or times, that they worked. Since the early books were (and are) so useless, Akiyama was right to say that the only true way to study fittings was by the close examination of the available pieces he would see. He felt that a classification of five or six "types" would "group" all the "early" tsuba into the various known varieties.