By Charles Gilman
IntroductionTunnelchess was my reason for first looking for Chess Variant websites - I was searching for names for its combined pieces. Having found that there were no standard names I came up with some of my own, Tunnelchess.
The BoardTunnelchess is a 3d variant modeled closely on FIDE Chess but each rank is a 4x4 square of cells and so there are twice as many files as in FIDE Chess (in other words, the board is a stack four high of 4 wide by 8 long boards). Ranks are numbered 1-8 as in FIDE Chess. Files are lettered a-d along the top, e-h next down, and so on.
The files form three groups. The Frame comprises outermost files a/d/m/p and so is occupied (in the simple version) by Rooks. The Wall comprises intermediate files b/c/e/h/i/l/n/o and so is occupied by minor pieces. The Tunnel, from which the game takes its name, comprises innermost files f/g/j/k and so is occupied by combined pieces, the nature of these pieces being what distinguishes this game from other inventors' variants on similar boards. The simple game is nevertheless playable with two standard sets, either distinguishable or, in some versions, identical but with one King and Queen marked. For games with 16 compound pieces it is worth getting two very distinct and very directional novelty sets, to avoid the potential false associations from e.g. Staunton sets. For the Pawnless version divide equally between both players each of the four groups of eight identical figures intended as Pawns!
Battle of Diagonals Tunnelchess
In all cases, as in most of my 3d and/or 4-player variants, there is no Castling.
A further variant using this board, Tunnelshogi, is sufficiently distinct to warrant its own page. That too is an extrapolation of radial pieces to 3d, but of Shogi and Mitregi rather than of FIDE Chess. Likewise Paired Piece Tunnelchess uses the board but drops the links betwen Frame/Wall/Tunnel files and the FIDE board's edge/intermediate/central ones. Finally there is a 4-player version of Intermediate Tunnelchess, Crosstunnel.
Pieces with no nonstandard diagonal (colloquially called triagonal) move have their FIDE names and images. The same symbols on their sides mark a pieces that changes move on crossing the Channel.
The linepiece on the nonstandard diagonal is the UNICORN of Raumschach, which is bound to a quarter of the board as the Bishop is to half of it.
The Rook+Unicorn compiund is a DUCHESS. Like the Queen the lady of a stately home, as represented by the more powerful component being the Rook.
The piece corresponding to the King is the same directions is a GRANDDUKE, after the ruler of a small state such as Luxembourg. Like the Knight this piece switches between the two Bishop bindings.
The Bishop+Unicorn compound is a GOVERNOR. The name derives from the British sovereign's title in the Anglican Church, but the piece is capturable. It is not colourbound as the bindings of its components are different.
Pieces combining all three types of radial direction have imperial titles. The piece corresponding to the King is an EMPEROR. The linepiece is an EMPRESS.
If you are using distinguishable sets and wish to be adventurous, you could try one type of Pawnlike piece on the Frame and Tunnel, and another on the Wall. My article Man and Beast 2: Shield Bearers lists some to choose from. The most obvious combinations are Pawn and Yeoman, Broker and Lowlander, and Warder and Highlander. Whichever you choose, in these variants they have an optional initial double-step noncapturing move and En Passant. Another option is to use a single type, the COHEIR invented for Paired Piece Tunnelchess. This piece is a restricted version of the Heir, which moves one step along any forward radial except the orthogonal. Whereas the Heir need only switch to a different file when it moves, the Coheir must switch to a differnt kind of file. It cannot switch between two Tunnel or two Wall files. Thus the b2 (White) Coheir can make its first move to a3, f3, or g3, but not c3 or e3; the f5 (Black) Coheir can move to a4, b4, c4, e4, or i4 but not g4, j4, or k4. This is a compromise between the full Heir and a divergent piece, and specific to Tunnelchess geometry. It is strong enough not to require an initial double-step move.
Minor variations to Simple Tunnelchess6 RANKS - if 128 cells seems cumbersome reduce it to 96 and remove the Pawn's initial double move. It also helps concentrate the power of the pieces. This variation can also be made to Intermediate Tunnelchess; surrounding either array so modified with extra files for a range of oblique leapers is the basis of Leapale.
Any one of the next three:
EQUESTRIAN - replacing the lower two Bishops with Knights. This makes checkmate slightly easier as Knights are disproportionally powerful in 3 dimensions, especially on a narrow board. Players have to remember what all minor pieces are, so highly distinguishable sets are recommended.
DRAGOON - replacing one group of Pawnlike pieces with Knights. This retains the strength of the standard back row, but adds the power of Knights. As Knights in Equestrian Tunnelchess (and FIDE Chess) can leap through the Pawn rank anyway, having them already on the front rank is not so great a further change. On the downside one must remember that large Pawns represent Knights and Knights Unicorns!
CAVALCADE - keeping all four Bishops but adding the Knight move to the weak Unicorn to give a piece which I term Cavalcade (as explained in Man and Beast 08: Diverse Directions). This piece is also of great use in my bipolar variants.
either one of the following two:
CROOKED-DIAGONAL - replacing Bishops with Boyscouts (90° Crooked Bishops) and Unicorns with Longrangescouts (110° Crooked Unicorns). Note that the turning angle is specified to indicate pieces tending toward the Rook, making them stronger on this kind of board. As Man and Beast 09: Mighty Like a Rose explains, this geometry also has Long-/Short-boyhexers (60° and 120° Crooked Bishops), which are duals of hex-board Crooked Rooks, and the Shortrangescout (70° Crooked Unicorn). The first might be an interesting alternative, tending toward the Sextorider (the 2:1:1 rider of MAB 05: Punning by Numbers), but the other two tend toward the Bishop itself, which is no help at all! Of course you could allow all angles which, as with Larry Lynn Smith's 3d leapers, gives stronger pieces still, or even have Straight-and-Crooked compounds, including for Rooks. You will be relieved to hear that the only Crooked Rook on a cubic-cell board is the 90° Girscout! Further options include whether and how to apply this to pieces which are already compound!
BILLIARD - using Reflecting Bishops and Reflecting Unicorns. Some example of the latter's reflections are required, as they change direction in one dimension when bouncing of the wall or the end of the tunnel (b3-e2-j1-o2 demonstrates both in that order), and in two when bouncing off the frame or the end of the wall (b2-e1-j2 demonstates the former, d2-b1-g2 the latter). An intermediate subvariant uses One-Reflection Bishops and One-Reflection Unicorns. either one of the following two:
BARE-EMPEROR - each player must retain at least one piece in addition to the Emperor (or 3 pieces including King and Grandduke if combined with split-ruler). This helps overcome the difficulty of checkmating. You can count Medinese baring (the barer in imminent danger of also being bared) as a win or a draw, both have been used historically in 2d games.
FIGUREHEAD - each player must retain at least two kinds of combined linepiece, these representing the "power behind the throne". As well as a Medinese question, there is the question of whether imminent Pawn promotion can avert defeat.
either one of the following two:
WIN BY MARRIAGE - a pun on "mate" from Doug Chatham's Bachelor Chess. Allow Pawn promotion to Empress, a piece notably absent from the array. Moving an Empress to a square orthogonally adjoining the Emperor is an additional means of winning. The Emperor must still be out of check but there is a "Medinese marriage" question, whether imminent capture of the Empress averts defeat.
SPLIT-RULER - replace the Emperor with a King (no nonstandard diagonal move), and Governor with a Grandduke - both with their consort at their side from the start! Checkmating either wins.