7971Definition: Coplanar Points and Straight Lines
Different points (and/or straight lines), which are incident to a plane are said to be coplanar. We also say that a plane $\alpha$ joins the points $A_1,A_2,\ldots$ or the straight lines $l_1,l_2,\ldots,$ or the points and straight lines write $$\alpha=A_1A_2\ldots=l_1l_2\ldots=A_1l_1,A2,\ldots$$
In the text to follow, we want to agree that the order of the straight lines or points written down in this notation does not play any role, for instance, for a coplanar point $P$ and a straight line $l$, the expressions $Pl$ and $lP$ shall denote the same plane.
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