Petersen–Morley theorem

In geometry, the Petersen–Morley theorem states that, if $a$, $b$, $c$ are three general skew lines in space, if $a'$, $b'$, $c'$ are the lines of shortest distance respectively for the pairs $(b,c)$, $(c,a)$ and $(a,b)$, and if $p$, $q$ and $r$ are the lines of shortest distance respectively for the pairs $(a,a')$, $(b,b')$ and $(c,c')$, then there is a single line meeting at right angles all of $p$, $q$, and $r$.

The theorem is named after Johannes Hjelmslev (who published his work on this result under his original name Johannes Trolle Petersen) and Frank Morley.