How does what3words build a grid of squares on the surface of the Earth?

What3words built a grid of squares of 3m2 on the earth. Does anyone know what is the algorithm ?

I started with this method: the range of longitude is [-180, 180] and the range of latitude is [-90,90]. If you cut them into equal intervals, you can have almost perfect squares near equator, but near the poles, they arenot squares, are they ?

After Gary's answer. I still want some precisions.

If between -85° and 85° (latitudes), the squares are in a grid, do I have to assume that the number of squares at a any latitude is the same (for me, when I say a grid, for each side of the square, there should be one and only one square)? If so, at the equator, we can have a square of 3mx3m, but at latitude 85°, it is a 0.26m x 0.26m then. (Because the parallel at latitude 85° is cos(85°) shorter.)

So, do I have to assume that it is not a perfect grid , since the area of the square always 3m x 3m at any latitude between -85° and 85°. But when I tried to zoom into a what3word map, I always see a perfect grid. How is it possible? Is it possible to see some deformations, directly on a map ?

@Gary, is it possible to see the deformations (near the poles) you mentionned in a map ? (it seems that we can't see on a google map). 