**5.4. Conventional coordinate types**

In the first FITS paper, Wells et al.
([1981])
listed a number
of "suggested values" for `CTYPE `*i*. Two of these have the
attribute that they can assume only integer coordinate values and that
the meaning of these integers is only by convention. These two axis
types are in wide-spread use and we wish to repeat their definition
here to extend their definitions and to reserve their names and
meanings.

The first conventional coordinate is `CTYPE ia`

The second conventional coordinate is `CTYPE ia`

Value | Symbol | Polarization |

1 | I | Standard Stokes unpolarized |

2 | Q | Standard Stokes linear |

3 | U | Standard Stokes linear |

4 | V | Standard Stokes circular |

-1 | RR | Right-right circular |

-2 | LL | Left-left circular |

-3 | RL | Right-left cross-circular |

-4 | LR | Left-right cross-circular |

-5 | XX | X parallel linear |

-6 | YY | Y parallel linear |

-7 | XY | XY cross linear |

-8 | YX | YX cross linear |