TheTablebaseWindow

What is a Tablebase?

According to Wikipedia, a tablebase

contains the game-theoretical value (win, loss, or draw) of each possible move in each possible position, and how many moves it would take to achieve that result with perfect play. Thus, the tablebase acts as an oracle, always providing the optimal moves. Typically the database records each possible position with certain pieces remaining on the board, and the best moves with White to move and with Black to move.

SCID can use Nalimov tablebases for 3, 4, and 5 men. These often end with the file suffix .nbw.emd or .nbb.emd.

NOTE. SCID's code for Nalimov tablebases is no longer developed or supported. Please configure your chess engine to directly access tablebases.

The Tablebase Window

You can get even more tablebase information about the current position by opening the Tablebase window ([Windows] menu, shortcut: Ctrl+Shift+=). This window shows the result with perfect play of all legal moves from the current position. The window has two main parts. The summary frame (on the left) shows which tablebases SCID found on your computer and a summary for each tablebase. The results frame (on the right) shows optimal results for all moves from the current position displayed in the main window.

The summary frame

The top part of the summary frame lets you select a particular tablebase. Those you have available are shown in blue and unavailable tablebases are shown in gray, but you can select any tablebase. The lower part of the summary frame shows summary information for the selected tablebase. (Not all tablebases have a summary recorded in SCID yet.)

The summary includes the frequency (how many games per million reach a position with this material, computed from a database of more than 600,000 master-level games), a longest mate for either side, and the number of mutual (or "reciprocal") zugzwangs. A mutual zugwang is a position where white to move draws and black to move loses, or where white to move loses and black to move draws, or where whoever moves loses.

For some tablebases with mutual zugzwangs, the summary also includes a list of all of the zugwang positions or a selection of them. A full list for every tablebase is not feasible since some tablebases have thousands of mutual zugzwangs.

You can set up a random position from the selected tablebase by pressing the Random button.

The results frame

The results frame is updated whenever the chessboard in the main window changes. The first line shows how many moves win (+), draw (=), lose (-), or have an unknown result (?). The rest of the frame gives a more detailed list of results, ranking them from shortest to longest mates, then draws, then longest to shortest losses. All distances are to checkmate.

The results board

In a tablebase position, it is often useful what the tablebase results would be if all the pieces in the current position were on their current squares but one particular piece was moved somewhere else. For example, you may want to determine how close a king has to be to a passed pawn to win or draw a particular position. In endgame books this information is often called the winning zone or drawing zone of a piece in a position.

You can find this information in Scid by pressing the button with an image of a chessboard, to show the results board in the tablebase window. When you press the left mouse button on any piece in this board, a symbol is drawn in each empty square showing what the tablebase result would be (with the same side to move as the current main window position) if the selected piece was on that square.

There are five different symbols a square can have: a white # means White wins; a black # means Black wins; a blue = means the position is drawn; a red X means the position is illegal (because the kings are adjacent or the side to move is giving check); and a red ? means the result is unknown because the necessary tablebase file is not available.

How Do I Select Tablebases?

To use tablebase files in SCID, simply set their directories by selecting Tablebase directory... from the [Options] menu. You can select up to 4 directories where your tablebase files are stored. You can press a ... button to the right of an entry to choose a file, to specify that the directory of that file should be used.

When a position found in a tablebase file is reached, the game information area (below the chessboard) will show tablebase information. You can configure the amount of information shown by clicking the right-mouse button in that area or selecting Game information from the [Options] menu. Selecting the "result and best moves" option gives the most useful information, but is much often slower than the "result only" option.