The left side of the figure shows the surface of a half-cylinder, with its axis oriented horizontally. On the surface, the constant N lines are axial, and the constant M lines are circumferential. A "diagonal" line is drawn on the cylinder's surface.

The right side of the figure shows the same half-cylinder, with two new surfaces drawn on its surface. One extends to the left, from the original diagonal line, to the left base of the cylinder. The other extends to the right, from the original diagonal line, to the right base of the cylinder.

In the new surfaces, the "axial" grid lines are at the same "circumferential" locations as the points of the original diagonal line. The "circumferential" grid lines in the new surfaces are equally spaced in the axial direction.