diff --git a/doc/source/tutorial.rst b/doc/source/tutorial.rst
index a1f742c7c74d1d6cd5d4d8345092a4d02074c15f..da90443a4408a62ac4515b862024a4ae2d4cd7e3 100644
--- a/doc/source/tutorial.rst
+++ b/doc/source/tutorial.rst
@@ -138,6 +138,7 @@ This may be performed using the standard GROMACS too `gmx gyrate`::
     gmx gyrate -f ref.xtc -s ref-for-rgyr.tpr -o ref-gyr.xvg
     gmx gyrate -f md.xtc -s md.tpr -o cg-gyr.xvg
 
+In both cases select the 36KB group as the one on which to perform teh calculation.
 These commands will calculate the radius of gyration for each trajectory frame for both the reference and CG simulations.
 The resulting `.xvg` files may be visualised using a graphing program such as `xmgrace` or compared in the same way as the bonded samples, using::