天下三作 Tenka Sansaku

What is Tenka Sansaku in Japanese Katana terminology?

"Tenka Sansaku" refers to the three swordsmiths, "Awataguchi Yoshimitsu", "Masamune", and "Gou Yoshihiro", who were selected in the sword ledger "Kyoho Meibutsucho" produced during the Edo period as being particularly excellent in the production of Japanese swords. The first volume of the Kyoho Meibutsucho introduces only the swords of these three.