Parallelepiped
From the figure, we can deduce that the magnitude of α is limited to 0° ≤ α < 90°. On the contrary, the vector b × c may form with a an internal angle β larger than 90° (0° ≤ β ≤ 180°). Namely, since  b × c is parallel to h, the value of β is either β = α or β = 180° − α. So