--- a
+++ b/doc/_includes/units.rst
@@ -0,0 +1,36 @@
+:orphan:
+
+Internal representation (units)
+===============================
+
+.. NOTE: part of this file is included in doc/manual/io.rst and
+   doc/overview/implementation.rst. Changes here are reflected there. If you
+   want to link to this content, link to :ref:`manual-units` for the manual or
+   :ref:`units` for the implementation page. The next line is a target for
+   :start-after: so we can omit what's above:
+   units-begin-content
+
+Irrespective of the units used in your manufacturer's format, when importing
+data, MNE-Python will always convert measurements to the same standard units.
+Thus the in-memory representation of data are always in:
+
+- Volts (eeg, eog, seeg, emg, ecg, bio, ecog, dbs)
+- Teslas (magnetometers)
+- Teslas/meter (gradiometers)
+- Amperes*meter (dipole fits, minimum-norm estimates, etc.)
+- Moles/liter ("molar"; fNIRS data: oxyhemoglobin (hbo), deoxyhemoglobin (hbr))
+- Arbitrary units (various derived unitless quantities)
+
+.. NOTE: this is a target for :end-before: units-end-of-list
+
+Note, however, that most MNE-Python plotting functions will scale the data when
+plotted to yield nice-looking axis annotations in a sensible range; for
+example, :meth:`mne.io.Raw.plot_psd` will convert teslas to femtoteslas (fT)
+and volts to microvolts (µV) when plotting MEG and EEG data.
+
+The units used in internal data representation are particularly important to
+remember when extracting data from MNE-Python objects and manipulating it
+outside MNE-Python (e.g., when using methods like :meth:`~mne.io.Raw.get_data`
+or :meth:`~mne.Epochs.to_data_frame` to convert data to :class:`NumPy arrays
+<numpy.ndarray>` or :class:`Pandas DataFrames <pandas.DataFrame>` for analysis
+or plotting with other Python modules).