**Notation**

A configuration in the plane is denoted by (*p*_{γ} *ℓ*_{π}), where *p* is the number of points, *ℓ* the number of lines, γ the number of lines per point, and π the number of points per line. These numbers necessarily satisfy the equation

as this product is the number of point-line incidences.

The notation (*p*_{γ} *ℓ*_{π}) does not determine a projective configuration up to incidence isomorphism. For instance, there exist three different (9_{3} 9_{3}) configurations: the Pappus configuration and two less notable configurations.

In some configurations, *p* = *ℓ* and γ = π. These are called *symmetric* configurations and the notation is often condensed to avoid repetition. For example (9_{3} 9_{3}) abbreviates to (9_{3}).

Read more about this topic: Configuration (geometry)

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