F7 Sacrifice

The "f7 sacrifice" is the common name for the complex strategy where white sacrifices a  queen or a  bishop sacrifice the f7 square of an early board, putting black's king in a very weak position.
This sacrifice is only possible on some standard variants (Standard, Standard - Turn Zero, etc.) and usually results in a forced softmate if the opponent did not defend against it.
The term "f2 sacrifice" is also used for when black does the same thing, landing their queen or bishop on an early f2 square.

The f7 sacrifice relies on the fact that the f7 pawn (and respectively f2 pawn) is only defended by either king on the starting position.
Successfully sacrificing a queen, princess or bishop on that square forces the opponent to capture that piece with their king, exposing it to several diagonals and  triagonals and leaving the `e8-h5` (resp. `e1-h4`) diagonal open.

## Performing it

### As White, before the f7 sacrifice

#### 1. e3

As white, you need to time travel with the queen or the bishop to `(L0T1)f7` or, if possible (but less likely to win), to `(L0T2)f7`.
For this, you can exploit one of the following triagonals and diagonals:

`(L0T5)b3-(L0T1)f7` (triagonal, with the queen)

`(L0T5)f3-(L0T1)f7` (diagonal, with the queen or a bishop)

`(L0T7)f1-(L0T1)f7` (diagonal, with the bishop)

*Note: some other, less common time travel possibilities are highlighted in "Defending against the f7 sac"*

On Standard, white can open with
`1. e3`, threatening to later put the queen on
`(L0T5)f3`. Black's immediate response is
`1.. Nf6`, blocking the threatened diagonal off and protecting `h5`.

If white then plays
`2. c3`, threatening to instead put the queen on
`(L0T5)b3`, then black could play
`2.. d5`, blocking off all paths leading to the
`(L0T1)f7` square and making the f7 sacrifice impossible.

#### 2. Bb5

To prevent that, white can stop
`2.. d5` from being played, by instead playing
`2. Bb5`, pinning the d-pawn.
In this position, black can only block the triagonal by playing
`2.. Nd5`. However, it quickly leads to a softmate, for instance with the following line:

```
[Board "Standard"]
[Mode "5D"]

1. e3 / Nf6
2. Bb5 / Nd5
3. Qh5 / g6
4. Qxd5 / e6
5. (0T5)Qd5>>x(0T3)f7+ / (1T3)Kxf7
6. (1T4)Qh5+ / (1T4)g6
7. (1T5)Qxd5*
```
To the right is a screenshot of the position, a few moves later.See on chessin5d.net | What is this?
This means that in this position, black must now prepare for a counter-sacrifice (see Defending against it).

Following are moves that make it hard or impossible for white to put their queen on `b3`:

#### 2.. c6

After
`2. Bb5`, black can play
`2.. c6`, which is considered the most optimal defense (because if white goes for the f7 sacrifice and black manages to get out of it, they will be down a bishop).

In this position, white could ignore the bishop and play
`3. c3 / cxb5 4. Qb3`, but black will then play Template:4.. Qb6, preparing to make the f7 sacrifice non-viable.

#### 2.. c5, 2.. a5 or 2.. Nc6

The idea of these moves is to later threaten the `b3` square, by playing
`3.. c4`,
`3.. a4` or
`Na5`.

## Defending against it

On Standard, black must play
`1.. Nf6` against any opening by white as their first move. Doing so cuts off the `(0T1)f7-(0T7)f1` diagonal and makes the f7 sacrifice not viable on later boards.
Black's strategy is to then play
`2.. d5` and
`3.. c6` or
`3.. Nc6`, blocking the `(0T1)f7-(0T5)b3` triagonal.
If black is allowed to play
`1.. Nf6 2.. d5 3.. c6`, then white cannot perform the f7 sacrifice (see prevention example below).

If white brings their bishop out with
`1. e3 / Nf6 2. Bb5` (pinning the `d7` pawn), black should threaten the bishop with
`2.. c6` and capture it if white then plays
`3. c3`.
In this position, black can get their queen out on `b6` and use it to defend themselves if white performs the f7 sacrifice.

If white brings their queen out with
`1. c3 / Nf6 2. Qa4` (also pinning the `d7` pawn), black should also respond with
`2.. c6`. Black can later play
`3.. d5`, which leaves the `(0T1)f7-(0T5)c3` triagonal open but prevents off f7 sacrifices to later boards.
In this position, black can also get their queen out on `b6` to defend themselves.

### Successful f7 prevention

Following are annotated screenshots of a game where black managed to play
`1.. Nf6`,
`2.. d5` and
`3.. c6`, fully preventing the f7 sacrifice.

```
[Board "Standard"]
[Mode "5D"]

1. e3 / Nf6
2. c3 / d5
3. Bb5 / c6
4. Qb3 / h6
```
In this position, white cannot perform the f7 sacrifice (see annotated screenshots below)See on chessin5d.net | What is this?
Position after 1. e3 / Nf6 2. c3 / d5 3. Bb5 / c6 4. Qb3 / h6Annotated diagonalAnnotated triagonal
*(TODO: more lines, more schemas showing the blocked n-agonals)*

## History

*(TODO: explain the history of the f7 sacrifice: discovery, Nh3, Samet's stream and the new variants. Maybe explain how T0 mitigates the threat, with examples where standard f7 sac results in black winning)*