# Cross-surface

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A **cross-surface** (or **real projective plane**) is a possible finite Life universe in the form of a real projective plane. The simplest way to do this is to use an m × n rectangle with the top edge twisted and joined to the bottom edge and the left edge twisted and joined to the right edge. It is similar to the Klein bottle except it has both pairs of sides twisted instead of just one.

## See also

## External links

- Real projective plane at Wikipedia
- Golly Help: Bounded Grids at the Golly website