Tetrad

Religious movements and cults are often founded on a tetrad of elements: a prophet, a prophecy, a book, and a revelation.

They took figures for the abundance of invasive weeds mapped according to the normal grid unit of the 'hectad', or 10 × 10 km square, and then looked at how abundant these species were mapped at a much finer scale – in 'tetrads', or 2 × 2 km squares, inside these hectads.