Late Koto Period Blades vs. Early Shinto Period Blades?

I think it probably has more to do with the fact so much more effort was put into balance into a war period blade. By the later samurai periods the swords were more hip-accessories and were used in duels but seldomly. Though yes many types and exceptions exist all over the place. Also remember many many koto and nambokucho blades became suriage or o-suriage and lost a lot of length from some of the monster blades that existed.

I can say that, owning both a shinshinto blade and a nambokucho blade the nambokucho has a narrower sugata with more fumbari than the shinshinto blade. But that hardly makes for a generalization.

Most modern replica are pretty generic late edo sugata or showato even... (again, exceptions exist, especially in the custom realms).

Not necessarily a full explanation but my own observations as to the inquiry.

This is the "stereotypical" shape guide by NBTHK.



But those are only stereotypical swords that were kinda defining the era.

There can be very little difference in some swords made 1570 vs. 1640 for example. It is often said by those that "bash" or "critique" Japanese swords that the sword changed very little and when you look at above diagram for blade shapes it is true. The large war swords that were in use from Kamakura to Muromachi were kinda the only ones really breaking the mold with their unusual size. Yes there were changes in mounting & size but when you look at bigger picture the difference for something like 400 years might not be too large.

The "problem" with Edo period swords is they saw very little fighting. Where as earlier periods had fighting and wars going on. I know people are hyped about samurai duels etc. during Edo period but it was not really common. Swordsmanship declined, sword making declined, armor making declined because there was no need for new swords or armors as the country was in peace for so long.

There is a difference in steel used in the old times and in the new times, new steel was better quality and more uniform in general (Some old sword purists might cut my head off from saying that but I think more modern methods produce higher quality in average, likewise modern tool steels are great for swords). Also new swords are generally polished less.

I'm no expert on how martial arts developed in Japan but as Paul Martin said in his article above that thrusting in dojo became much more important and thicker profile is good for thrusting while thinner is better for cutting.

I think in general that "koto feels light, shinto feels heavy" saying is geared towards the notion that old swords were of superior make. Even though I focus my collecting on old swords I do not fully think like that. Good swords and bad swords were made during all periods. It is amazing to see for example c. 1000 year old swords from legendary smiths but they have much legend attached to them and in reality they most likely are not superior to for example modern made swords. However it is remarkable what the smiths achieved 1000 years ago when you think the level of technology and knowledge back then. That legend about old swords is part of the charm in collecting swords, at least for me.