diff --git a/composer.json b/composer.json
index ad55823eb..8cb7ac49a 100644
--- a/composer.json
+++ b/composer.json
@@ -1,5 +1,5 @@
 {
-    "name": "php-coord/php-coord",
+    "name": "stevegoddard/php-coord",
     "description": "PHPCoord is a set of PHP functions for handling various co-ordinate systems and converting between them",
     "keywords": ["coord","geo", "wgs84", "osgb36", "latitude", "longitude", "grid ref", "utm", "itm" ,"gps"],
     "homepage": "https://github.com/dvdoug/PHPCoord",
diff --git a/src/OSRef.php b/src/OSRef.php
index 494c820a3..c95bc97b1 100644
--- a/src/OSRef.php
+++ b/src/OSRef.php
@@ -19,6 +19,7 @@
 class OSRef extends TransverseMercator
 {
     private const GRID_LETTERS = 'VWXYZQRSTULMNOPFGHJKABCDE';
+    private const GRID_LETTERS_TETRAD = 'AFKQVBGLRWCHMSXDINTYEJPUZ';
 
     /**
      * @return RefEll
@@ -91,7 +92,10 @@ public function __construct($x, $y, $z = 0)
      */
     public static function fromGridReference(string $ref): self
     {
-        if (strlen($ref) % 2 !== 0) {
+        $tetrad = false;
+        if(strlen($ref) === 5) {
+            $tetrad = true;
+        } elseif (strlen($ref) % 2 !== 0) {
             throw new LengthException('Grid ref must be an even number of characters');
         }
 
@@ -103,20 +107,33 @@ public static function fromGridReference(string $ref): self
         $minorEasting = strpos(self::GRID_LETTERS, $ref[1]) % 5 * 100000;
         $minorNorthing = (floor(strpos(self::GRID_LETTERS, $ref[1]) / 5)) * 100000;
 
+        //tetrad letter is 2km grid sq. THE GRID HAS A DIFFERENT ORIENTATION - starts bottom left and runs bottom to top. Includes I but no O.
+        $tetradEasting = 0;
+        $tetradNorthing = 0;
+        if ($tetrad) {
+            $tetradEasting = strpos(self::GRID_LETTERS_TETRAD, $ref[4]) % 5 * 2000;
+            $tetradNorthing = (floor(strpos(self::GRID_LETTERS_TETRAD, $ref[4]) / 5)) * 2000;
+        }
+
         //numbers are a division of that square into smaller and smaller pieces
-        $numericPortion = substr($ref, 2);
-        $numericPortionSize = strlen($numericPortion) / 2;
+        if ($tetrad) {
+            $numericPortion = substr($ref, 2, 2);
+            $numericPortionSize = strlen($numericPortion) / 2;
+        } else {
+            $numericPortion = substr($ref, 2);
+            $numericPortionSize = strlen($numericPortion) / 2;
+        }
         $gridSizeInMetres = 1 * (10 ** (5 - $numericPortionSize));
 
-        $easting = $majorEasting + $minorEasting + (substr($numericPortion, 0, $numericPortionSize) * $gridSizeInMetres);
-        $northing = $majorNorthing + $minorNorthing + (substr($numericPortion, -$numericPortionSize, $numericPortionSize) * $gridSizeInMetres);
+        $easting = $majorEasting + $minorEasting + $tetradEasting + (substr($numericPortion, 0, $numericPortionSize) * $gridSizeInMetres);
+        $northing = $majorNorthing + $minorNorthing + $tetradNorthing + (substr($numericPortion, -$numericPortionSize, $numericPortionSize) * $gridSizeInMetres);
 
         return new static((int) $easting, (int) $northing);
     }
 
     /**
      * Convert this grid reference into a grid reference string of a
-     * given length (2, 4, 6, 8 or 10) including the two-character
+     * given length (2, 5, 4, 6, 8 or 10) including the two-character
      * designation for the 100km square. e.g. TG514131.
      *
      * @param int $length
@@ -125,11 +142,19 @@ public static function fromGridReference(string $ref): self
      */
     public function toGridReference(int $length): string
     {
-        if ($length % 2 !== 0) {
+        $tetrad = false;
+        if($length === 5) {
+            $tetrad = true;
+        } elseif ($length % 2 !== 0) {
             throw new LengthException('Chosen length must be an even number');
         }
 
-        $halfLength = $length / 2;
+        // manually set tetrad half length
+        if ($tetrad) {
+            $halfLength = 1;
+        } else {
+            $halfLength = $length / 2;
+        }
 
         $easting = str_pad((string) $this->x, 6, '0', STR_PAD_LEFT);
         $northing = str_pad((string) $this->y, 6, '0', STR_PAD_LEFT);
@@ -141,13 +166,26 @@ public function toGridReference(int $length): string
         $majorLetterIndex = (int) (5 * $majorSquaresNorth + $majorSquaresEast);
         $majorLetter = substr(self::GRID_LETTERS, $majorLetterIndex, 1);
 
-        //second (minor) letter is 100km grid sq, origin at 0,0 of this square
-        $minorSquaresEast = $easting[0] % 5;
-        $minorSquaresNorth = $northing[0] % 5;
-        $minorLetterIndex = (5 * $minorSquaresNorth + $minorSquaresEast);
+        //second (minor) letter is 100km grid sq, origin at 0,0 of this square - work from the modulus
+        $majorSquaresEastModulus = $adjustedX % 500000;
+        $majorSquaresNorthModulus = $adjustedY % 500000;
+        $minorSquaresEast = floor($majorSquaresEastModulus / 100000);
+        $minorSquaresNorth = floor($majorSquaresNorthModulus / 100000);
+        $minorLetterIndex = (int) (5 * $minorSquaresNorth + $minorSquaresEast);
         $minorLetter = substr(self::GRID_LETTERS, $minorLetterIndex, 1);
 
-        return $majorLetter . $minorLetter . substr($easting, 1, $halfLength) . substr($northing, 1, $halfLength);
+        // tetrad
+        $tetradLetter = null;
+        if ($tetrad) {
+            $minorSquaresEastModulus = $majorSquaresEastModulus % 10000;
+            $minorSquaresNorthModulus = $majorSquaresNorthModulus % 10000;
+            $tetradSquaresEast = floor($minorSquaresEastModulus / 2000);
+            $tetradSquaresNorth = floor($minorSquaresNorthModulus / 2000);
+            $tetradLetterIndex = (int) (5 * $tetradSquaresNorth + $tetradSquaresEast);
+            $tetradLetter = substr(self::GRID_LETTERS_TETRAD, $tetradLetterIndex, 1);
+        }
+
+        return $majorLetter . $minorLetter . substr($easting, 1, $halfLength) . substr($northing, 1, $halfLength) . $tetradLetter;
     }
 
     /**
diff --git a/tests/OSRefTest.php b/tests/OSRefTest.php
index 778bd5443..62025d3e1 100644
--- a/tests/OSRefTest.php
+++ b/tests/OSRefTest.php
@@ -147,6 +147,23 @@ public function testFromSixFigureString2(): void
         self::assertEquals($expected, $OSRef->__toString());
     }
 
+    public function testFromTetradString(): void
+    {
+        $OSRef = OSRef::fromGridReference('SO24G');
+        $expected = '(322000, 242000)';
+
+        self::assertEquals($expected, $OSRef->__toString());
+    }
+
+    public function testToTetradString(): void
+    {
+        $OSRef = new OSRef(216604, 771209);
+
+        $expected = 'NN17Q';
+
+        self::assertEquals($expected, $OSRef->toGridReference(5));
+    }
+
     public function testIssue7(): void
     {
         $osRef = new OSRef(322000, 241000);