Opposite colour occupancy
Pieces of opposite colours can share the same field. If they are benign as with two pawns there are no rules to observe when the second piece joins the first.
There are situations when a pairing can take place where the two pieces are not actively attacking each other whilst in the field but could have done so when entering (eg two knights). In this case, the second piece when entering the field must declare its intention to share the field – (Je partage – I share).
The same rule applies to pieces that whilst sharing would actively be attacking its fellow field companion. These essentially involve rooks, bishops and queens. In these cases the square to be occupied must be available within the scope of that pieces movement (knights excepted). This is illustrated in the diagram left where the g6 rook can occupy the g8 square and share the field whereas as the g11 rook cannot pass the g9 rook to occupy g8.