Two lines<span> in the same three-dimensional space that </span>do not intersect<span> need </span>not<span> </span><span>be</span>parallel<span>. Only </span>if they<span> are in a common plane are </span>they<span> called </span>parallel<span>; otherwise </span>they<span>are</span><span> called skew </span>lines<span>.</span>