Learn four- and six-figure Ordnance Survey grid references on a visual grid — remember: eastings (across) first, northings (up) second.
How grid references work: a 4-figure reference (e.g. 2437) names one 1km square: 24 = eastings (read across, left to right), 37 = northings (read up, bottom to top). A 6-figure reference (e.g. 243375) divides that square into tenths — 243 is 3/10 of the way across, 375 is 5/10 of the way up. Always say eastings before northings: "along the corridor, up the stairs".