Changes in the Shape of a Japanese Sword (1000 years)

In the world of Japanese swords, there are a number of significant time periods to discover. It is quite fascinating to note that just by examining the overal shape and curvature of a particular sword, one can make accurate assumptions as to the age of the sword as shown in the image below.
Copyright NBTHK Japan. Click image for larger view.
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NBTHK-Copyright-Japanese-Swords-Evolution