Each block shown on the map below is a tetrad. A tetrad is made up of an area that covers 2k x 2k . Each tetrad is assigned a letter so that when viewed on a 10k x 10k square, it looks like the layout on the right.

As you run the cursor over each tetrad in the main map, a label will appear with an assigned code. You can use the grid to the right to work out the gridref for the tetrad. Just substitute the letter with firstly the number below it, and then the number to the left of it: i.e. 58A would be SJ5080 and 66M would be SJ 6464.

 
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    0    2   4     6    8

Blue squares = tetrads already allocated

Blank squares = tetrads awaiting allocation

29X 39C 18Z 28E 18Y 29F 28D 28C 28J 28I 28H 29K 28P 28N 28G 28M 28L 28K 27P 28Q 28R 28S 28T 28U 29Q 29R 27U 27T 27S 29W 29V 28Z 28Y 28X 28W 28V 27Z 27Y 27X 27W 39B 39A 38E 38D 38C 38B 38A 37E 37D 37C 37B 39F 38J 38I 38H 38G 38F 37J 37I 37H 37G 38M 38L 38K 37P 37N 37M 37L 37K 38S 38R 37U 37T 37S 37R 37Q 36U 37X 37Y 37Z 37W 37V 36Z 36Y 36X 36W 36R 36L 36V 36Q 36K 35U 35Z 46A 46F 46K 46Q 65T 66V 65Z 65P 66Q 65S 66R 66W 66S 66L 66T 76B 76G 76L 76R 76W 76X 76V 75Z 75Y 76Y 76Z 77V 75X 86B 86G 86H 86F 85J 85I 86L 86I 86J 87F 87G 59V 87I 86T 86R 86U 87Q 87R 86N 87S 86S 86Y 87T 87N 87Y 97D 87U 88Q 97I 87P 97J 97H 97G 97F 96J 97N 98F 98G 98H 97M 97S 97X 97L 97K 96P 96N 97P 98K 98L 98M 97Y 97Z 98V 98W 98X 97W 97V 96Z 96Y 97T 97U 98Q 98R 98S 97R 97Q 96U 96T 96I 96M 96D 96H 96C 96E 97A 97B 97C 97E 86X 86W 86V 86Z 87V 87W 87X 88V 98A 98B 98C 88L 88R 88G 88B 78W 78R 78S 78Q 77U 77T 77S 77R 77Q 77K 87A 76U 77F 77A 77B 77C 77D 77I 77N 77M 77P 78K 78L 78M 78H 78G 78I 78F 77J 77E 78A 77Z 87E 87J 88F 78V 88K 88A 87D 87C 87B 77Y 77X 77W 87L 87H 77L 77G 76J 76P 76E 76D 66X 76C 76H 76M 66Y 66Z 67V 65Y 75D 75I 75N 75T 75U 76Q 75S 75R 75Q 75K 75L 75M 75P 76K 76F 75J 75E 85E 86E 86D 86C 86A 85D 85C 85P 85U 86Q 85N 86M 86K 85H 67Q 67K 67F 67A 57V 57Q 65U 57R 57S 57T 57U 56U 56T 56S 56R 56W 66B 66G 66F 65J 65I 65H 65G 65F 64J 64P 64I 64E 54Z 54U 64U 64Z 65V 65W 65X 64Y 64X 64W 64V 74E 74J 74D 75A 75B 75C 74C 74B 74I 74N 64T 64N 64M 64L 64K 65K 65L 65M 65N 66K 66M 66N 66P 67L 67M 67N 67P 68K 68F 68A 58V 58Q 68G 68H 68I 68J 69F 69K 69A 69Q 58Z 59W 58Y 58X 68C 58S 58M 58L 58N 58P 59K 59L 59M 59S 69H 69M 69S 69T 69N 69R 69L 69G 69B 68E 68D 68B 58W 67E 67D 67C 67H 57X 57Y 57Z 57W 56Z 56Y 56X 66C 66H 66I 66J 67B 67R 67W 67S 67T 67U 67Z 68Q 68R 68S 68T 68U 68P 68N 68M 68L 68W 78B 78C 78D 68Y 68X 68Z 69V 69W 58D 58E 58C 58B 58A 57E 57J 47Z 47U 47P 47J 47E 47D 47C 47B 47A 46E 46D 46I 46N 46T 46Y 56D 56I 56N 56H 56G 56F 55J 55I 55H 55G 55L 55R 55B 55A 54E 54D 54C 54B 45W 45R 45L 45K 44P 44N 44M 88M 45S 45T 45X 45M 45N 45H 45I 45J 46G 46L 46R 46B 46H 46W 56B 56A 56C 55E 55D 55C 45Z 55P 55U 55Z 45U 45P 45E 45D 45C 46C 46M 46S 46X 46Z 47V 47W 47X 47Y 57B 57G 57L 47R 47L 47G 47F 47H 47I 47N 47T 47S 47M 47K 46P 46J 46U 56E 56J 57F 57H 57I 58F 56P 66E 66D 66U 56M 56L 56K 56Q 56V 66A 65E 65D 65C 65B 65A 64D 54Y 54T 64C 64B 64G 54W 54X 55V 55W 55X 55Y 55T 55N 55M 55K 54P 54N 54M 45Y 46V 44Z 45V 44U 44Y 44X 44S 54H 54S 64H 64F 64A 64Q 64R 64S 65Q 65R 75G 75H 75F 74H 55S 55Q 45F 45G 44J 44I 45Q 44T 54I 54J 55F 58G 48W 48Y 48X 48R 48V 48Q 47Q 57A 57K 57P 58K 57D 57C 67X 68V 67Y 77H 67I 67J 58R 58T 58U 59Q 59R 58I 58H 78X 88H 76T 76N 76I 76S 76A 87M 87K 86P 57N 57M 67G

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