diff --git a/info/Commands/fortify.t b/info/Commands/fortify.t
index 9a1e59c5..539f66bc 100644
--- a/info/Commands/fortify.t
+++ b/info/Commands/fortify.t
@@ -18,7 +18,7 @@ A fully fortified unit is twice as hard to hurt as it normally
 would be, and is twice as strong on the defense.
 .s1
 Units fortifying in a sector containing an engineering unit
-are better at it. Experiment and find out.
+are better at it: they get 50% more fortification per mobility.
 .s1
 If a unit moves or retreats, it loses all fortification value.
 A unit that reacts to defend a sector, on the other hand, does NOT
@@ -30,4 +30,12 @@ Note that fortification will not help a unit fight Guerrillas in
 an occupied sector (see info Guerrilla).  Also note that fortifying
 a unit does not affect it's mission status.
 .s1
+Units with maximum mobility automatically use further mobility gains
+to fortify.  For example, when a unit with 120 of 127 mobility gains
+60 mobility, its mobility increases to the maximum 127, and it uses 53
+mobility to fortify.
+.s1
+Units requiring fuel (if option FUEL is enabled), do not fortify
+automatically, as this would burn fuel.
+.s1
 .SA "bomb, Unit-types, LandUnits"
diff --git a/info/Concepts/Mobility.t b/info/Concepts/Mobility.t
index 8ace4f23..95dfa4d1 100644
--- a/info/Concepts/Mobility.t
+++ b/info/Concepts/Mobility.t
@@ -36,8 +36,9 @@ For each 2 points of efficiency a land unit looses fighting rebels at
 the update, it looses one point of mobility.
 
 3. Fortification
-To fortify ("dig in") a land unit costs one point of mobility per
-point of fortification.
+Fortifying a land unit ("dig in") costs one point of mobility per
+point of fortification.  An engineer cuts the mobility cost by one
+third.  See "info fortify".
 
 4. Converting civs
 Security units are charged 10 mobility every time civs are converted