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You can use WinBoard (on Windows) or XBoard (Linux, Mac) with Sjaak II as an engine to play Chess variants. Sjaak II is a configurable multi-variant engine, where the user can define its own variants in a text file through a rather straightforward description. (A file with instructions how to do that, with many examples, is included with Sjaak II.)

E.g. to define Judkins Shogi, the game-definition file would have to contain:

#################
# Judkins Shogi #
#################
Variant: Judkins Shogi (6x6)
Board: 6x6
FEN: "rbnsgk/5p/6/6/P5/KGSNBR"
XBoard pieces: "PNBR.S...G.++++.+Kpnbr.s...g.++++.+k"
Zone: white_promotion = a5,b5,c5,d5,e5,f5,a6,b6,c6,d6,e6,f6
Zone: black_promotion = a2,b2,c2,d2,e2,f2,a1,b1,c1,d1,e1,f1

# Define the pieces
Piece: Knight
Move: aleap (1,2)|(-1,2)
Symbol: "N", "N,n"
Promotion: white_promotion, black_promotion, "+"
Value: 250

Piece: Bishop
Move: slide (D,A)
Symbol: "B", "B,b"
Promotion: white_promotion, black_promotion, "+"
Value: 575

Piece: Silver general
Move: aleap (0,1)|(1,1)|(1,-1)|(-1,-1)|(-1,1)
Symbol: "S", "S,s"
Promotion: white_promotion, black_promotion, "+"
Value: 375

Piece: Gold general
Move: aleap (1,0)|(-1,0)|(0,1)|(0,-1)|(1,1)|(-1,1)
Symbol: "G", "G,g"
Value: 450

Piece: Rook
Move: slide (H,V)
Symbol: "R", "R,r"
Promotion: white_promotion, black_promotion, "+"
Value: 650

Piece: Pawn
Move: step N
Symbol: "P", "P,p"
Promotion: white_promotion, black_promotion, "+"
Value: 80

Piece: Promoted Knight
Move: aleap (1,0)|(-1,0)|(0,1)|(0,-1)|(1,1)|(-1,1)
Symbol: "+N", "+N,+n"
Value: 500

Piece: Dragon Horse
Move: slide (D,A)
Move: leap (1, 0)
Symbol: "+B", "+B,+b"
Value: 825

Piece: Promoted Silver
Move: aleap (1,0)|(-1,0)|(0,1)|(0,-1)|(1,1)|(-1,1)
Symbol: "+S", "+S,+s"
Value: 490

Piece: Dragon King
Move: slide (H,V)
Move: leap (1, 1)
Symbol: "+R", "+R,+r"
Value: 950

Piece: Promoted Pawn
Move: aleap (1,0)|(-1,0)|(0,1)|(0,-1)|(1,1)|(-1,1)
Symbol: "+P", "+P,+p"
Flags: drop_no_mate, drop_one_file
Value: 530


Piece: King
Move: leap (0,1)|(1,1)
Symbol: "K", "K,k"
Flags: royal

Rule: keep capture, allow drops