Blue Moon lays all fifty-two cards face up in four rows of fourteen, then moves the four Aces to the head of the rows — leaving four gaps where they came from. Those gaps are the entire game. A gap is filled by the card one rank higher and of the same suit as whatever sits immediately to its left, and moving that card opens a new gap somewhere else. Nothing is hidden and nothing is random after the deal, so every position is a puzzle you can actually solve. Free in your browser, no sign-up.
Goal: make every row read Ace to King in a single suit.
With four gaps and no hidden cards, the whole game is about which gap you spend next — because every move you make destroys one gap and creates another somewhere you may not want it.
Blue Moon belongs to the grid family usually named after Montana or Gaps — games with no piles at all, where the board is a rectangle of slots and the holes in it are the mechanism. The PySol Fan Club collection parameterises the whole group with three numbers, and Blue Moon's are RLEN, RSTEP, RBASE = 56, 14, 0: fifty-six slots in rows of fourteen, with a leftmost gap accepting an Ace.
The difference from Gaps is larger than the extra column suggests. Gaps is Montana, which takes the four Aces out of play entirely: its rows are twelve cards long, they start at a two, and a leftmost gap is a wildcard any two may fill. Blue Moon keeps all fifty-two cards on the board, its rows are thirteen cards long behind an Ace, and those Aces are the anchors rather than being absent. The two boards look alike at a glance and the opening decisions are not the same.
Its other close relative in this catalogue is Paganini, David Parlett's two-deck version, where the Aces stay in the grid and may be moved to a leftmost slot whenever the player chooses — and where there is no redeal at all.
One honest note on difficulty. No published win rate for Blue Moon that we can find carries a method or a sample size, so this page quotes none. Structurally the two redeals are what make the family playable at all: without them a single badly placed King can end a game in a dozen moves.
Gaps is Montana, which removes the four Aces from play entirely — its rows are twelve cards long and start at a two. Blue Moon keeps all 52 cards, seats the Aces at the head of each row, and its rows are thirteen cards long.
The card one rank higher and of the same suit as whatever sits immediately to the gap's left. A gap in the leftmost column takes an Ace instead, since it has no left neighbour to match.
A gap immediately to the right of another gap has no left neighbour, so nothing matches it. The same is true of a gap to the right of a King — there is no card above a King, so that slot is dead for good.
You get two, and they are only offered when no move remains at all. Every card already seated in an unbroken run from its row's Ace stays put; everything else is gathered, shuffled and laid out again.
None. All 52 are face up from the first second and nothing is random after the deal, so every position can be reasoned about completely rather than guessed at.
Making a legal move without looking at where the new gap will appear. Every fill opens a gap where that card used to be, and opening one beside a King or another gap wastes the move entirely.