@tantalor is technically correct; meridians and parallels intersect at right angles, even with respect to the (idealized) surface of the earth. Proof sketch: if the angles involved in a crossing were not all 90°, then either the crossing would not be symmetric about the meridian (it is), or one or both lines must have a sharp "kink" (i.e., have a non-smooth first derivative) at the intersection (neither does).

However, parallels, while smooth, are not straight, they "curl" toward the nearest pole (i.e., non-zero second derivative, relative to the earth's surface). This accounts for the non-"rectangular" shape of quadrangles and can be replicated on a 2D plane.

Yes with regard to the surface of the earth.

Take the lines tangent to the meridian & parallel at the point of intersection. Observe these lines are perpendicular. Hence the meridian & parallel meet at a right angle.

You’re right, I apologize for my smartass answer.

What about the ‘boxes’ where the top edge is, er, the point at one of the poles?

Yeah not the pole of course, but you can cut them off at at ±89.999 degrees and it still works.

> Recall that all meridians intersect at the poles.

How is that relevant regarding “Parallels and meridians always meet at 90 degrees“?