Japan uses the American YDS for lead routes; French grade is the reference when travelling to Europe.
The Dan-Kyu system carries cultural weight not found in V or French grades. Progressing by decreasing "Kyu" and eventually reaching "Dan" holds significance beyond a difficulty number.
The Dan-Kyu system originated in the Edo period with Go and Shogi, later spreading to Judo, Kendo, tea ceremony, and calligraphy. "Kyu" means a person in training — smaller numbers mean higher skill. Breaking through 1-Kyu earns "Shodan," one who has mastered the basics and stands at the start of true exploration. Japanese bouldering pioneers applied this system to outdoor problems, treating climbing as a path ("Do") with its own spirituality.
Whether one can climb Shodan is a clear boundary among climbers. Completing a Shodan is spoken of as proof of fundamental body movement, finger strength, and the mental fortitude to face the rock earnestly — not just physical power.
V10 is extremely difficult to reach as an extension of casual training — it demands strength, focus, holding power, and precision meshing at a high level. If Shodan means joining the ranks of serious climbers, Sandan is the next massive goal into the territory of strong, top-tier climbers.