## Related pagesCoordinatization of projective planesAffine coordinate planes |

**Definition.** A coordinate frame of
is a tupel (o,e,U,V) where o,e are two points of
and U,V are two parallel classes of
such that
.

The line is called the diagonal of the coordinate frame (o,e,U,V). We have the following simple lemma.

**Lemma.** The map

is a bijection, called the coordinate map corresponding to the coordinate frame (o,e,U,V).

Moreover, we have the following theorem.

**Theorem.** Let K be the diagonal of a coordinate frame (o,e,U,V).
Then the map

defines a ternary field (K,T), and the coordinate map is an isomorphism of onto the affine plane over (K,T).

In particular, each affine plane is isomorphic to the affine plane over some ternary field. But note that distinct coordinate frames of an affine plane usually yield non isomorphic ternary fields.

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