diff --git a/docs/design_guide.rst b/docs/design_guide.rst
index 75825893a9..7a8c76b507 100644
--- a/docs/design_guide.rst
+++ b/docs/design_guide.rst
@@ -520,7 +520,9 @@ properties.
 +-----------------------+-----------------------+-------------------------------------------------------------------------+
 | ``temperature``       | float                 | degrees centigrade                                                      |
 +-----------------------+-----------------------+-------------------------------------------------------------------------+
-| ``eCO2``              | float                 | equivalent CO2 in ppm                                                   |
+| ``CO2``               | float                 | measured CO2 in ppm                                                     |
++-----------------------+-----------------------+-------------------------------------------------------------------------+
+| ``eCO2``              | float                 | equivalent/estimated CO2 in ppm (estimated from some other measurement) |
 +-----------------------+-----------------------+-------------------------------------------------------------------------+
 | ``TVOC``              | float                 | Total Volatile Organic Compounds in ppb                                 |
 +-----------------------+-----------------------+-------------------------------------------------------------------------+