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#### Built-in Vector Identifiers

There are several built-in vector identifiers. You can use them to specify values or to create expressions but you cannot re-declare them to change their values. They are:

VECTOR_BUILT-IN_IDENT:
`
  x
` | `y` | `z` | `t` | `u` | `v`

All built-in vector identifiers never change value. They are defined as though the following lines were at the start of every scene.

     #declare x = <1, 0, 0>;

     #declare y = <0, 1, 0>;

     #declare z = <0, 0, 1>;

     #declare t = <0, 0, 0, 1>;

     #declare u = <1, 0>;

     #declare v = <0, 1>;

The built-in vector identifiers `x`, `y`, and `z` provide much greater readability for your scene files when used in vector expressions. For example....

     plane { y, 1}    // The normal vector is obviously "y".

     plane { <0,1,0>, 1} // This is harder to read.

     translate 5*x    // Move 5 units in the "x" direction.

     translate <5,0,0>  // This is less obvious.

An expression like `5*x` evaluates to `5*<1,0,0>` or `<5,0,0>`.

Similarly `u` and `v` may be used in 2D vectors. When using 4D vectors you should use `x`, `y`, `z`, and `t` and POV-Ray will promote `x`, `y`, and `z` to 4D when used where 4D is required.

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