Not every floor is perfectly horizontal. To make sure you lay horizontal tiles, it's good to lay your first row of tiles one row above floor level. To do this, measure your tile height, and mark a point on the wall that is 20mm below that height. Then use your spirit level to draw a horizontal line, known as a “datum line”, at that height all the way around the room. The height is reduced by 20mm just in case there is a low point in the floor that needs covering.