diff --git a/doc/lattice.html b/doc/lattice.html
index f0c45f21a2..e3dd79db8a 100644
--- a/doc/lattice.html
+++ b/doc/lattice.html
@@ -121,8 +121,10 @@ and a3 are 3 orthogonal unit vectors (edges of a unit cube).  But you
 can specify them to be of any length and non-orthogonal to each other,
 so that they describe a tilted parallelepiped.  Via the <I>basis</I>
 keyword you add atoms, one at a time, to the unit cell.  Its arguments
-are fractional coordinates (0.0 <= x,y,z < 1.0), so that a value of
-0.5 means a position half-way across the unit cell in that dimension.
+are fractional coordinates (0.0 <= x,y,z < 1.0).  The position vector
+x of a basis atom within the unit cell is thus a linear combination of
+the the unit cell's 3 edge vectors, i.e. x = bx a1 + by a2 + bz a3,
+where bx,by,bz are the 3 values specified for the <I>basis</I> keyword.
 </P>
 <HR>
 
diff --git a/doc/lattice.txt b/doc/lattice.txt
index ec622d3434..38f38ae58b 100644
--- a/doc/lattice.txt
+++ b/doc/lattice.txt
@@ -113,8 +113,10 @@ and a3 are 3 orthogonal unit vectors (edges of a unit cube).  But you
 can specify them to be of any length and non-orthogonal to each other,
 so that they describe a tilted parallelepiped.  Via the {basis}
 keyword you add atoms, one at a time, to the unit cell.  Its arguments
-are fractional coordinates (0.0 <= x,y,z < 1.0), so that a value of
-0.5 means a position half-way across the unit cell in that dimension.
+are fractional coordinates (0.0 <= x,y,z < 1.0).  The position vector
+x of a basis atom within the unit cell is thus a linear combination of
+the the unit cell's 3 edge vectors, i.e. x = bx a1 + by a2 + bz a3,
+where bx,by,bz are the 3 values specified for the {basis} keyword.
 
 :line