Are the lines slanted or parallel?
The horizontal lines in this image appear to be sloping, but in reality they're parallel to one another.
Why does it work? Although it's easy to see the mortar line between two black tiles or two white tiles,
it's much harder to see the mortar line between a white tile and a black one. Your brain fills in the gap
by seeing it as part of either a white or black tile. This, in turn, makes the tiles look wider at one end
than at the other, creating the illusion of a series of wedge-shaped tiles, which makes the lines appear to slant.