\nparallelThe symbol \nparallel is used to denote that two lines or vectors are not parallel to each other. It is the negation of the parallel relation, which is typically represented by the symbol \parallel.
Line AB is not parallel to line CD.
AB \nparallel CDVector \vec{u} is not parallel to vector \vec{v}.
\vec{u} \nparallel \vec{v}Line l_1 is not parallel to line l_2.
l_1 \nparallel l_2