Clarify the conventions for rotations in left and right handed coordinate system

base_classes/NXtransformations.nxdl.xml

@@ -95,7 +95,7 @@
 		group is equally possible.
 
 
-		Following the section on the general dscription of axis in NXtransformations is a section which
+		Following the section on the general description of axis in NXtransformations is a section which
 		documents the fields commonly used within NeXus for positioning purposes and their meaning. Whenever
 		there is a need for positioning a beam line component please use the existing names. Use as many fields
 		as needed in order to position the component. Feel free to add more axis if required. In the description
@@ -111,7 +111,7 @@
 
 		NOTE
 
-		NXtransformations follows the **active** transformation convention. This means that the transformation describes
+		``NXtransformations`` follows the **active** transformation convention. This means that the transformation describes
 		how an object is moved or rotated within the coordinate system. In other words, the transformation
 		actively changes the position or orientation of the object itself. This is in contrast to a **passive** transformation,
 		which changes the frame of reference or coordinate system, while the object remains fixed. In case it is needed 
@@ -136,7 +136,7 @@
 				in which case the values are angular rotations around the axis.
 
 				If this attribute is omitted, this is an axis for which there
-				is no motion to be specifies, such as the direction of gravity,
+				is no motion to be specified, such as the direction of gravity,
 				or the direction to the source, or a basis vector of a
 				coordinate frame. In this case the value of the ``AXISNAME`` field
 				is not used and can be set to the number ``NaN``.
@@ -151,16 +151,32 @@
 			<doc>
 				Three values that define the axis for this transformation.
 				The axis should be normalized to unit length, making it
-				dimensionless.  For ``rotation`` axes, the direction should be
-				chosen for a right-handed rotation with increasing angle.
+				dimensionless.
 				For ``translation`` axes the direction should be chosen for
 				increasing displacement. For general axes, an appropriate direction
 				should be chosen.
 
-				Note that if the ``NXtransformation`` depends on a coordinate system
-				(i.e., its ``depends_on`` attribute points to an instance :ref:`NXcoordinate_system`),
-				the rotation is right handed in that coordinate system, even if the the coordinate system
-				itself is left-handed (as defined by the determinant of its base vectors).
+				By default, for ``rotation`` axes that do not explicitly depend on a coordinate system,
+				the direction should be chosen for a right-handed rotation with increasing angle. Note,
+				McStas is a right handed coordinate system.
+
+				If the ``NXtransformation`` depends on a coordinate system (i.e., its ``depends_on`` 
+				attribute (or a ``depends_on`` further up the transformation chain) points to an instance
+				of :ref:`NXcoordinate_system`), the rotation convention is the same as the handedness
+				of the coordinate system (as defined by the determinant of its base vectors):
+
+				* Rotations in left-handed coordinate systems are left-handed (i.e., they follow the 
+				  left-hand grip rule). In a left-handed coordinate system, positive rotation about an
+				  axis is clockwise when looking from a point on the positive axis towards
+				  its origin (from infinity towards the origin).
+
+				* Rotations in right-handed coordinate systems are right-handed (i.e., they follow the 
+				  right-hand grip rule). In a right-handed coordinate system, positive rotation about an
+				  axis is counter-clockwise when looking from a point on the positive axis towards
+				  its origin (from infinity towards the origin).
+
+				Note that by using this convention, the transformation matrices in both left- and
+				right-handed coordinate systems are the same.
 			</doc>
 			<dimensions rank="1">
 				<dim index="1" value="3" />