diff --git a/raillabel/format/quaternion.py b/raillabel/format/quaternion.py
index ecff8d0..366747a 100644
--- a/raillabel/format/quaternion.py
+++ b/raillabel/format/quaternion.py
@@ -8,25 +8,24 @@
 
 @dataclass
 class Quaternion:
-    """A quaternion.
-
-    Parameters
-    ----------
-    x: float or int
-        The x component of the quaternion.
-    y: float or int
-        The y component of the quaternion.
-    z: float or int
-        The z component of the quaternion.
-    w: float or int
-        The w component of the quaternion.
-
-    """
+    """A rotation represented by a quaternion."""
 
     x: float
+    "The x component of the quaternion."
+
     y: float
+    "The y component of the quaternion."
+
     z: float
+    "The z component of the quaternion."
+
     w: float
+    "The omega component of the quaternion."
+
+    @classmethod
+    def from_json(cls, json: tuple[float, float, float, float]) -> Quaternion:
+        """Construct an instant of this class from RailLabel JSON data."""
+        return Quaternion(x=json[0], y=json[1], z=json[2], w=json[3])
 
     @classmethod
     def fromdict(cls, data_dict: dict) -> Quaternion:
diff --git a/raillabel/format/transform.py b/raillabel/format/transform.py
index 22c2cc3..c8cd55a 100644
--- a/raillabel/format/transform.py
+++ b/raillabel/format/transform.py
@@ -26,12 +26,7 @@ def from_json(cls, json: JSONTransformData) -> Transform:
         """Construct an instant of this class from RailLabel JSON data."""
         return Transform(
             position=Point3d.from_json(json.translation),
-            quaternion=Quaternion(
-                x=json.quaternion[0],
-                y=json.quaternion[1],
-                z=json.quaternion[2],
-                w=json.quaternion[3],
-            ),
+            quaternion=Quaternion.from_json(json.quaternion),
         )
 
     @classmethod
diff --git a/tests/test_raillabel/format/test_quaternion.py b/tests/test_raillabel/format/test_quaternion.py
index 8291881..d508e85 100644
--- a/tests/test_raillabel/format/test_quaternion.py
+++ b/tests/test_raillabel/format/test_quaternion.py
@@ -29,6 +29,11 @@ def quaternion() -> dict:
 # == Tests ============================
 
 
+def test_from_json(quaternion, quaternion_dict):
+    actual = Quaternion.from_json(quaternion_dict)
+    assert actual == quaternion
+
+
 def test_fromdict():
     quaternion = Quaternion.fromdict([0.75318325, -0.10270147, 0.21430262, -0.61338551])