## 7969**Definition**: Incidence

By **incidence**, we mean in projective geometry that a point and a straight line, or a point and a plane, or a straight line and a plane are subsets and supersets of each other in the set-theoretic sense. In other words, if $A$ is a point, $l$ is a straight line and $\alpha$ is a plane, then

- $A$ and $l$ are
**incident**(or “$A$**lies on**$l$”, or “$l$**passes through**$A$”), if and only if $A\subset l,$ - $A$ and $\alpha$ are
**incident**(or “$A$**lies on**$\alpha$”, or “$\alpha$**passes through**$A$”), if and only if $A\subset \alpha,$ - $l$ and $\alpha$ are
**incident**(or “$l$**lies on**$\alpha$”, or “$\alpha$**passes through**$l$”), if and only if $l\subset \alpha.$

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