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Historical note (skip if you like):

In this interesting puzzle @TSLF asks for the "maximum number of black and white pieces that are involved in a checkmate position".

Originally, they left much open to interpretation, and interpret me and a handful of others did!

My interpretation did not exactly win, but I actually quite like it, so I decided to make a separate question of it.

Challenge:

Construct a legal position where white to move mates in one, subject to:

1. The mate is unique. Update: This riddle smith is not above the law as layed down by the World Federation for Chess Composition in their Handbook of Chess Composition, article 13(2) with footnote 16 (thanks @trolley813) where it says: "[...] except that in the final move a promotion into different pieces having partially the same power (for example queen/rook or queen/bishop) may be tolerated.
2. Removing any piece except a king results in a legal position (white to move, i.e. white king may be in check, black king mustn't).

An "accessory to regicide" is any piece which when removed leaves a position where white can no longer mate in one.

Your task: Maximise the number of accessories.

Bonus:

A "witness" is a non-king piece that is not an accessory. For those not well versed in the fine art of regicide: A perfect regicide is one where there are no witnesses. Let us define the regicide score as number of accessories - 3 x number of witnesses

Bonus task: Maximise the regicide score.

Almost Perfectly valid example per article 13(2) and footnote 16 of the Handbook of Chess Comoposition:

Note board is rotated. White pawns move down.

20 accessories (The kings and the black queens except for the one directly adjacent to her king do not count.) Four witnesses. Score: 8. This is not a valid example because there are technically two mates: gxh8B# and gxh8Q#

Historical note (skip if you like):

In this interesting puzzle @TSLF asks for the "maximum number of black and white pieces that are involved in a checkmate position".

Originally, they left much open to interpretation, and interpret me and a handful of others did!

My interpretation did not exactly win, but I actually quite like it, so I decided to make a separate question of it.

Challenge:

Construct a legal position where white to move mates in one, subject to:

1. The mate is unique. Update: This riddle smith is not above the law as layed down by the World Federation for Chess Composition in their Handbook of Chess Composition, article 13(2) with footnote 16 (thanks @trolley813) where it says: "[...] except that in the final move a promotion into different pieces having partially the same power (for example queen/rook or queen/bishop) may be tolerated.
2. Removing any piece except a king results in a legal position (white to move, i.e. white king may be in check, black king mustn't).

An "accessory to regicide" is any piece which when removed leaves a position where white can no longer mate in one.

Your task: Maximise the number of accessories.

Bonus:

A "witness" is a non-king piece that is not an accessory. For those not well versed in the fine art of regicide: A perfect regicide is one where there are no witnesses. Let us define the regicide score as number of accessories - 3 x number of witnesses

Bonus task: Maximise the regicide score.

Perfectly valid example per article 13(2) and footnote 16 of the Handbook of Chess Comoposition:Faulty example (thanks @Magma who points out that the Nc8 cannot be removed without creating an illegal position.)

Note board is rotated. White pawns move down.

20 accessories (The kings and the black queens except for the one directly adjacent to her king do not count.) Four witnesses. Score: 8. This is not a valid example because there are technically two mates: gxh8B# and gxh8Q#

Historical note (skip if you like):

In this interesting puzzle @TSLF asks for the "maximum number of black and white pieces that are involved in a checkmate position".

Originally, they left much open to interpretation, and interpret me and a handful of others did!

My interpretation did not exactly win, but I actually quite like it, so I decided to make a separate question of it.

Challenge:

Construct a legal position where white to move mates in one, subject to:

1. The mate is unique.
2. Removing any piece except a king results in a legal position (white to move, i.e. white king may be in check, black king mustn't).

An "accessory to regicide" is any piece which when removed leaves a position where white can no longer mate in one.

Your task: Maximise the number of accessories.

Bonus:

A "witness" is a non-king piece that is not an accessory. For those not well versed in the fine art of regicide: A perfect regicide is one where there are no witnesses. Let us define the regicide score as number of accessories - 3 x number of witnesses

Bonus task: Maximise the regicide score.

Almost example:

Note board is rotated. White pawns move down.

20 accessories (The kings and the black queens except for the one directly adjacent to her king do not count.) Four witnesses. Score: 8. This is not a valid example because there are technically two mates: gxh8B# and gxh8Q#

Historical note (skip if you like):

In this interesting puzzle @TSLF asks for the "maximum number of black and white pieces that are involved in a checkmate position".

Originally, they left much open to interpretation, and interpret me and a handful of others did!

My interpretation did not exactly win, but I actually quite like it, so I decided to make a separate question of it.

Challenge:

Construct a legal position where white to move mates in one, subject to:

1. The mate is unique. Update: This riddle smith is not above the law as layed down by the World Federation for Chess Composition in their Handbook of Chess Composition, article 13(2) with footnote 16 (thanks @trolley813) where it says: "[...] except that in the final move a promotion into different pieces having partially the same power (for example queen/rook or queen/bishop) may be tolerated.
2. Removing any piece except a king results in a legal position (white to move, i.e. white king may be in check, black king mustn't).

An "accessory to regicide" is any piece which when removed leaves a position where white can no longer mate in one.

Your task: Maximise the number of accessories.

Bonus:

A "witness" is a non-king piece that is not an accessory. For those not well versed in the fine art of regicide: A perfect regicide is one where there are no witnesses. Let us define the regicide score as number of accessories - 3 x number of witnesses

Bonus task: Maximise the regicide score.

Almost Perfectly valid example per article 13(2) and footnote 16 of the Handbook of Chess Comoposition:

Note board is rotated. White pawns move down.

20 accessories (The kings and the black queens except for the one directly adjacent to her king do not count.) Four witnesses. Score: 8. This is not a valid example because there are technically two mates: gxh8B# and gxh8Q#

Historical note (skip if you like):

In this interesting puzzle @TSLF asks for the "maximum number of black and white pieces that are involved in a checkmate position".

Originally, they left much open to interpretation, and interpret me and a handful of others did!

My interpretation did not exactly win, but I actually quite like it, so I decided to make a separate question of it.

Challenge:

Construct a legal position where white to move mates in one, subject to:

1. The mate is unique.
2. Removing any piece except a king results in a legal position (white to move, i.e. white king may be in check, black king mustn't).

An "accessory to regicide" is any piece which when removed leaves a position where white can no longer mate in one.

Your task  : Maximise the number of accessories.

Almost example:

Note board is rotated. White pawns move down.

20 accessories (The kings and the black queens except for the one directly adjacent to her king do not count.) This is not a valid example because there are technically two mates: gxh8B# and gxh8Q#

Historical note (skip if you like):

In this interesting puzzle @TSLF asks for the "maximum number of black and white pieces that are involved in a checkmate position".

Originally, they left much open to interpretation, and interpret me and a handful of others did!

My interpretation did not exactly win, but I actually quite like it, so I decided to make a separate question of it.

Challenge:

Construct a legal position where white to move mates in one, subject to:

1. The mate is unique.
2. Removing any piece except a king results in a legal position (white to move, i.e. white king may be in check, black king mustn't).

An "accessory to regicide" is any piece which when removed leaves a position where white can no longer mate in one.

Your task: Maximise the number of accessories.

Bonus:

A "witness" is a non-king piece that is not an accessory. For those not well versed in the fine art of regicide: A perfect regicide is one where there are no witnesses. Let us define the regicide score as number of accessories - 3 x number of witnesses

Bonus task: Maximise the regicide score.

Almost example:

Note board is rotated. White pawns move down.

20 accessories (The kings and the black queens except for the one directly adjacent to her king do not count.) Four witnesses. Score: 8. This is not a valid example because there are technically two mates: gxh8B# and gxh8Q#