Where do the letters come from?
In Hampshire, all tetrads have a format of SUnnX (or SZn9X along the coast), where SUnn is the Ordnance Survey 10-km square (hectad) reference, and X is the tetrad letter. The tetrads are given letters A, B, C, etc starting from the SW (bottom left) & moving up. We leave out letter “O” so as to avoid confusion with zero, and to leave us with 25 letters. Such tetrad lettering is used by most British biodiversity recorders, and is often referred to as EJPUZ-DINTY notation because thats how it reads along the top of the 10-km square! See the diagram elsewhere.
