diff --git a/format_specs.md b/format_specs.md
index a553613..1a63df2 100644
--- a/format_specs.md
+++ b/format_specs.md
@@ -26,3 +26,20 @@ The body of the puzzle starts consistently at offset `0x34`.
 |Solution|`0x34`|`width * height`|Based on the height and width, the solution is written out as ASCII.|
 |Layout|(end of the solution)|`width * height`|The layout of the grid is defined with `.` used as "empty cell"|
 |Clues|(end of the layout)|???|`NUL` separated strings containing each of the clues, not numbered.|
+
+#### Clue numbering and tagging
+
+The file doesn't contain information in which cells correspond to across and down clues. This information can be derived from the puzzle layout and the set of clues available:
+
+```
+FOREACH cell IN grid:
+    IF cell has a non-empty top neighbour
+    THEN the cell is part of its top neighbour's down word
+    ELSE the cell is part of the next available down word
+
+    IF cell as a non-empty left neighbour
+    THEN the cell is part of its left neighbour's across word 
+    ELSE the cell is part of the next available across word
+```
+
+By picking off clues from the across and down clue lists, assigning them using the algorithm above and numbering them as you go, you can build the correct cell <> clue mappings.