If one does a image search for monoclinic, one finds contradictions left and right on mathematics for they are claiming the Monoclinic crystal is a rectangular prism, when a rectangular prism has 6 faces of rectangles. They are claiming a monoclinic can have two 90 degree angles and be a parallelepiped.
So I think what has happened in the history of gemology, physics and chemistry, is that no-one in math came to sort these things out and tell them what is impossible and possible.
You can have a object whose alpha and gamma angles = 90 degrees but beta angle is not 90 degrees but you cannot call it a rectangular prism, nor even a parallelepiped, for such a object has no mathematical name. What it is, is a stacking of thin rectangular solids whose edges are therefor notched.
You cannot be drawing pictures of monoclinic crystals with straight line edges, but rather notched all along the edges.
You cannot use the terms rectangular prisms, parallelogram, parallelepiped nor draw pictures of them when describing Monoclinic Crystal Structure.