diff --git a/docs/source/usersguide/input.rst b/docs/source/usersguide/input.rst
index 751da5e82..917a834e5 100644
--- a/docs/source/usersguide/input.rst
+++ b/docs/source/usersguide/input.rst
@@ -48,6 +48,43 @@ files are called:
 Geometry Specification -- geometry.xml
 --------------------------------------
 
+Types of surfaces:
+
+x-plane
+  A plane perpendicular to the x axis, i.e. a surface of the form x - x0
+  = 0. The coefficients specified are "x0".
+
+y-plane
+  A plane perpendicular to the y axis, i.e. a surface of the form y - y0
+  = 0. The coefficients specified are "y0".
+
+z-plane
+  A plane perpendicular to the z axis, i.e. a surface of the form z - z0
+  = 0. The coefficients specified are "z0".
+
+plane
+  An arbitrary plane of the form A*x + B*y + C*z = D. The coefficients
+  specified are "A B C D".
+
+x-cylinder
+  An infinite cylinder whose length is paralle to the x-axis. This is a
+  quadratic surface of the form (y - y0)^2 + (z - z0)^2 = R^2. The coefficients
+  specified are "y0 z0 R".
+
+y-cylinder
+  An infinite cylinder whose length is paralle to the y-axis. This is a
+  quadratic surface of the form (x - x0)^2 + (z - z0)^2 = R^2. The coefficients
+  specified are "x0 z0 R".
+
+z-cylinder
+  An infinite cylinder whose length is paralle to the z-axis. This is a
+  quadratic surface of the form (x - x0)^2 + (y - y0)^2 = R^2. The coefficients
+  specified are "x0 y0 R".
+
+sphere 
+  A sphere of the form (x - x0)^2 + (y - y0)^2 + (z - z0)^2 = R^2. The
+  coefficients specified are "x0 y0 z0 R".
+
 ----------------------------------------
 Materials Specification -- materials.xml
 ----------------------------------------