In order to have the sides of the frame parallel, we could use a pair of x and y angles that added give 90 degrees. In such case, the picture frame will not only have the sides parallel, (a parallelogram), but in the shape of a rectangle.
Rectangle is a figure that has 4 equal internal angles (all of them 90dgrees) and four sides which are parallel in pairs.
A very nice option which would also simplify the cutting of the frame material would be the use of x = y = 45 degrees.
That way , the same piece of wood could be use to extract the four sides, and with minimum waste.
Notice that if we don't use 45 degrees, then the width of the wood used for the frame would have to be different for the top and for the sides
As we can see, if the width of the wood for the frame is the same in the top portion and in the side (vertical) portion of the frame, then we need anges that are exactly 45 degrees, because we are dealing with "isosceles" triangles that have one angle 90 degrees and the other two angles (opposite to sides of the SAME length) have to add to 90 degrees and be of the SAME measure. Then they must be 45 degrees each.
2 x + 90 = 180
2 x = 180 - 90
x = 90 / 2
x =45 degrees.