Game Phases

The phase describes where a game is in its overall arc. It's a property of the whole board, so both players share the same phase. BackgammonDB recognizes five.

Phase is one axis of position tagging; for the structural, per-player label see Position Types.

The five phases, defined

A position is in exactly one phase. The classifier checks them in order and takes the first that fits; Middle is the catch-all that holds everything else.

Phase	Definition	A position enters it when	Reversible?
Opening	The first few plies, still near the standard starting setup, before any blood is drawn.	All three hold: it's within the first few moves of the game (the first four plies), and little ground has been covered (total pip movement from the 334-pip start is modest), and nobody is on the bar. The moment a checker is hit, the opening is over — even a first-move hit drops the game straight into the middle.	No — latches shut
Middle	The contact-heavy heart of the game, where most decisions live: making points, slotting, priming, anchoring.	It is none of the other four phases (the catch-all).	—
Endgame (contact)	Low combined pip count, but a hit is still possible — one shot can swing everything.	The combined pip count falls below a low threshold (with a dead-band — see below).	Yes — the only two-way boundary
Race	The two armies have passed each other; no hit is possible for either side. A pure pip race.	Contact becomes impossible for both sides.	No — terminal
Bear-off	At least one side has all fifteen checkers home with no contact remaining, and is taking them off.	≥ 1 side is fully home with no contact left.	No — terminal

The endgame dead-band

To keep the endgame label from flickering it uses a dead-band: a position enters the endgame when the combined pip count falls below a low threshold, and only leaves it (back to middle) when the count climbs back above a higher one. The gap between the two means a single hit's pip swing won't bounce the phase back and forth — only a genuine reopening, with checkers sent well back, returns the game to the middle.

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Why phase can step backward (just not on a wobble)

Most phase transitions are one-way: the opening latches shut, and race and bear-off are terminal. The one boundary that can move both directions is middle ↔ endgame-contact — because a hit sends a checker ~25 pips back to the bar and can lift a low-pip position back into middle-game territory. The dead-band described above is what stops that boundary from twitching every time the pip count crosses a single line.

In the match review, phase changes show up as section dividers in the action list and as vertical markers on the progression graph, so you can see at a glance where a game moved from one phase to the next.