Leagues are listed here from lowest (entry-level) to highest difficulty:
Bronze
Silver
Gold
Sapphire
Ruby
Emerald
Amethyst
Pearl
Obsidian
Diamond
Mission accomplished!
Potential Discovery or Invention
Here is a chess position with multiply-twinned pieces during my match with Oscar.
Here is a chess position when Oscar (WHITE) called "Check", but he should have declared "Checkmate".
However, my geological engineer/exploration geologist daughter Évelyne, who is a chess master, pointed out that my BLACK pawn could have done "en passant" to eat the WHITE pawn in this situation.
Potential Discovery or Invention
Duolingo offers free language courses, but Energy and Gems deplete rather quickly. Shop > Special Offers contains "Free Chest" which provides random quantities of Energy and Gems for slow replenishment. By repeatedly tapping button "Free Chest" every 3 minutes or so to watch ads while listening to music videos on YouTube, I can accumulate enough Energy and Gems for the next language session.
