The π-bonds between C1 and C2 are perpendicular to that of C2 and C3 by πp - πp overlapping. Therefore, the hydrogen attached to C1 and those attached to C2 are in different planes (ie, perpendicular). σ-bonds between C1 - C2 and C2 - C3 are sp2 - sp and sp - sp2 overlapping on their axes.