Which five pairs add up to eleven? Ace and ten, two and nine, three and eight, four and seven, five and six. Learn those five and you already know the arithmetic of this two-deck patience.
One hundred and four cards sit on the table: twelve piles of eight laid out in a wide arc, and eight loose cards resting underneath them. Only the top card of each pile can be touched, so every removal uncovers something new and quietly rewrites what is possible next.
Number cards leave in pairs that total eleven, suits ignored. Court cards work differently. A Jack, a Queen and a King go together, three at a time, in any mixture of suits. Two cards of the same rank never pair, which catches most players out once.
The eight loose cards below are the tempting part. They are always available, but nothing ever replaces them. Spend them early and the endgame has no give left in it; hold them back and a stuck board often reopens.
Everything is face-up from the first second, so nothing is hidden and nothing is guessed. Play without a connection, step back through a hundred moves, follow your run of clears in the statistics, and read it all in any of 33 languages.
Half Moon solitaire: match pairs to clear the board. Free and offline.