Of course! This is an excellent question that breaks down the concept of a "game end" in two very different and interesting ways. Here's a detailed explanation covering both perspectives.

1. Game Endings by Rule/Category

A chess game can end in two primary outcomes: a win for one player or a draw. Here are the specific rules that lead to these results, as defined by FIDE (the International Chess Federation).

A Win for One Player

A player wins when their opponent loses. This can happen in the following ways:

• Checkmate: This is the most definitive way to win. The opposing king is under attack (in "check") and there is no legal move to escape the attack. The game ends immediately.
• Resignation: A player may voluntarily concede the game at any time if they believe their position is hopeless. This is done by saying "I resign," stopping the clock, or tipping over their king.
• Loss on Time (Timeout): In games with a time control, a player loses if they run out of time on their clock. There is one crucial exception: if the player who ran out of time has an opponent with insufficient mating material (see below), the game is a draw.
• Forfeit: A player can lose for violating the rules of the game. This can include things like cheating, using a phone, refusing to comply with an arbiter's ruling, or (in some tournaments) making a second or third illegal move.

A Draw

A game is a draw when neither player wins. This is also known as a "half-point" for each player in tournaments.

• Stalemate: It is a player's turn to move, their king is not in check, but they have no legal moves. The game ends immediately in a draw. This is a common trap in endgames.
• Draw by Agreement: Both players may simply agree that the position is equal or that neither can force a win. One player offers a draw, and the other accepts.
• Threefold Repetition: If the exact same board position occurs three times with the same player to move, the game is a draw. The position must be identical in every respect, including whose turn it is, castling rights, and any possible en passant captures. A player must claim this draw.
• The Fifty-Move Rule: If 50 consecutive moves have been made by each player without a pawn move or a capture, a player can claim a draw.
• Insufficient Mating Material: The game is drawn if a position is reached where neither player could possibly deliver checkmate by any sequence of legal moves. This happens automatically. Common examples include:
  • King vs. King
  • King and Bishop vs. King
  • King and Knight vs. King
  • King and Bishop vs. King and Bishop on the same color squares.
• Fivefold Repetition & Seventy-five-move Rule: These are more recent FIDE rules that make draws automatic, requiring no claim from a player.
  • Fivefold Repetition: If the same position occurs five times, the game is automatically a draw.
  • Seventy-five-move Rule: If 75 consecutive moves pass without a pawn move or capture, the game is automatically a draw.

2. Game Endings by Final Position

This is a fascinating and profoundly complex question in computational mathematics. The short answer is:

There is no exact, known number for the total possible final positions in chess.

Here’s a breakdown of why this is so difficult to calculate and what we do know.

The Core Problem: Reachability

The main challenge isn't just counting all possible checkmate or stalemate positions. The real problem is determining which of those positions are reachable from the starting position through a legal sequence of moves.

1. Total Legal Positions: The estimated number of unique and legal chess positions is enormous, often cited to be between 10⁴³ and 10⁴⁷. This is a combinatorial calculation based on placing up to 32 pieces on 64 squares.
2. Defining a "Final Position": A final position is a legal position where the game has just ended. This means it must be a position of:
   • Checkmate
   • Stalemate
   • A state of insufficient material to continue.
3. The "Reachable" Constraint: This is the killer. A board setup might look like a legal checkmate, but it might be impossible to get there from the starting position. For example, a position with White's king in checkmate by a Black pawn on g2, but White's own pawns are still on f2 and h2, is impossible. That Black pawn could never have legally reached g2.

What Has Been Studied?

While a total count is out of reach, this problem has been explored in a limited fashion:

• Endgame Tablebases: Computer scientists have solved this problem for a small number of pieces. An endgame tablebase is a database of all possible positions with a certain number of pieces (e.g., 7 or fewer). For every single one of these positions, the tablebase provides the definitive outcome (win, loss, or draw) and the optimal move. For these small subsets of chess, we do know the exact number of final positions. The 7-piece Lomonosov tablebase, for example, contains all outcomes for every position with 7 or fewer pieces on the board.
• Mathematical Bounds: We can establish some loose upper bounds. The number of final positions is obviously less than the total number of legal positions (~10⁴⁷). It is also much larger than the number of positions in the 7-piece tablebases.
• Distinction from Game Complexity: Don't confuse this with the Shannon Number (~10¹²⁰), which is an estimate of the game-tree complexity of chess—the number of possible unique games that can be played. The number of final positions is much smaller, as many different games can lead to the same final position.

Summary of Complexity

Calculating the exact number of reachable final positions would essentially require a program to explore the entire game tree of chess and mark every terminal node (a final position), which is computationally impossible with current technology. The problem shifts from pure board-state combinatorics to a graph theory problem of determining reachability within a graph of unimaginable size.