diff --git a/doc/building_on_microworld.md b/doc/building_on_microworld.md
index e7155a5..1819f7e 100644
--- a/doc/building_on_microworld.md
+++ b/doc/building_on_microworld.md
@@ -2,7 +2,7 @@
 
 In [Settling a Game World](Settling-a-game-world.html) I intended that a world should be populated by setting agents - settlers - to explore the map and select places to settle according to particular rules. In the meantime, I've built [MicroWorld](https://github.com/simon-brooke/mw-ui), a rule driven cellular automaton which makes a reasonably good job of modelling human settlement. It works, and I now plan to use it, as detailed in this note; but there are issues.
 
-First and formost, it's slow, and both processor and memory hungry. That means that at continent scale, a cell of one kilometre square is the minimum size which is really possible, which isn't small enough to create a settlement map of the density that a game will need. Even with 1km cells, even on the most powerful machines I have access to, a continent-size map will take many days to run.
+First and foremost, it's slow, and both processor and memory hungry. That means that at continent scale, a cell of one kilometre square is the minimum size which is really possible, which isn't small enough to create a settlement map of the density that a game will need. Even with 1 km cells, even on the most powerful machines I have access to, a continent-size map will take many days to run.
 
 Of course it would be possible to do a run at one km scale top identify areas which would support settlement, and then to do a run on a ten metre grid on each of those areas to more precisely plot settlement. That's an idea which I haven't yet explored, which might prove fruitful.