Forty Thieves
Family: Two-Deck (Double Deck)
A hard, two-deck, 10-column classic where skill matters greatly.
Forty Thieves deals two decks (104 cards) into 10 columns of four cards each. The basic rules are same-suit descending building in the tableau and moving only one card at a time.
You must fill 8 foundations by suit from Ace to King, and since you can move only one card at a time, planning is extremely important. Its low win rate makes it a real challenge.
It's also nicknamed "Napoleon," and several derivatives exist that ease the rules slightly.
Related variants
Two-Deck (Double Deck)
Spider
Two decks (104 cards); complete same-suit K→A sequences.
Klondike FamilyCanfield
A tough variant defined by a 13-card reserve and a random starting rank.
Klondike FamilyKlondike
The most widely known standard Solitaire. It's the default game on this site.