diff --git a/graphicle/transform.py b/graphicle/transform.py
index 70e6eda..e5847db 100644
--- a/graphicle/transform.py
+++ b/graphicle/transform.py
@@ -18,13 +18,25 @@
 
 
 class SphericalAngle(ty.NamedTuple):
-    """Pair of inclination and azimuthal angles, respectively."""
+    """Pair of inclination and azimuthal angles, respectively.
+
+    :group: transform
+
+    .. versionadded:: 0.4.0
+    """
 
     theta: float
     phi: float
 
 
 class SphericalAxis(ty.NamedTuple):
+    """Axis vector in 3D cartesian coordinates.
+
+    :group: transform
+
+    .. versionadded:: 0.4.0
+    """
+
     x: float
     y: float
     z: float
@@ -56,6 +68,10 @@ def soft_hard_axis(momenta: gcl.MomentumArray) -> SphericalAngle:
     """Calculates the axis defined by the plane swept out between the
     hardest and softest particles in ``momenta``.
 
+    :group: transform
+
+    .. versionadded:: 0.4.0
+
     Parameters
     ----------
     momenta : MomentumArray
@@ -80,6 +96,10 @@ def rotation_matrix(
     """Computes the matrix operator to rotate a 3D vector with respect
     to an arbitrary ``axis`` by a given ``angle``.
 
+    :group: transform
+
+    .. versionadded:: 0.4.0
+
     Parameters
     ----------
     angle : float
@@ -123,6 +143,10 @@ def split_momentum(
     Energy and 3-momentum is conserved. Hardness and collinearity of the
     split are determined by ``z`` and ``angle``.
 
+    :group: transform
+
+    .. versionadded:: 0.4.0
+
     Parameters
     ----------
     momentum : MomentumArray
@@ -171,6 +195,10 @@ def split_hardest(
     Hardness and collinearity of the split are determined by function
     parameters.
 
+    :group: transform
+
+    .. versionadded:: 0.4.0
+
     Parameters
     ----------
     momenta : MomentumArray