Grid square notations used in BTO surveys and datasets

The grid system that BTO uses for the Bird Atlases of Britain & Ireland, and other datasets. Whilst the main notation used for 10-km squares follows Ordnance Survey and Ordnance Survey Ireland nomenclature and is in common usage, those for tetrads, 20-km squares and 50-km squares are less widely used outside BTO and other wildlife recording schemes and societies. When using this document, please can you acknowledge the BTO using the reference on this page. --- We would also greatly appreciate if you could fill out this very short form to tell us how you intend to use these data. Thanks in advance!