diff --git a/mkdocs.yml b/mkdocs.yml
index 4b9e3449bd..97526d9730 100644
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -29,6 +29,7 @@ nav:
           - BIDS Derivatives: derivatives/introduction.md
           - Common data types and metadata: derivatives/common-data-types.md
           - Imaging data types: derivatives/imaging.md
+          - Diffusion MRI: derivatives/diffusion-derivatives.md
           - Templates and atlases: derivatives/atlas.md
       - Longitudinal and multi-site studies: longitudinal-and-multi-site-studies.md
       - Glossary: glossary.md
@@ -157,6 +158,7 @@ plugins:
         "05-derivatives/01-introduction.md": "derivatives/introduction.md"
         "05-derivatives/02-common-data-types.md": "derivatives/common-data-types.md"
         "05-derivatives/03-imaging.md": "derivatives/imaging.md"
+        "05-derivatives/03-diffusion.md": "derivatives/diffusion-derivatives.md"
         "06-longitudinal-and-multi-site-studies.md": "longitudinal-and-multi-site-studies.md"
         "07-extensions.md": "extensions.md"
         "99-appendices/14-glossary.md": "glossary.md"
diff --git a/src/derivatives/diffusion-derivatives.md b/src/derivatives/diffusion-derivatives.md
new file mode 100644
index 0000000000..7752429e8d
--- /dev/null
+++ b/src/derivatives/diffusion-derivatives.md
@@ -0,0 +1,31 @@
+# Diffusion derivatives
+
+## Diffusion-based tractography
+
+Tractography based on diffusion-weighted MRI data is stored as streamlines in
+the TRX file format:
+
+```Text
+<pipeline_name>/
+    sub-<label>/
+        [ses-<label>/]
+            <modality/
+                <source-entities>[space-<space>]_[tract-<tract name>]_[track-<tracking method>]_tractogram.trx
+                <source-entities>[space-<space>]_[tract-<tract name>]_[track-<tracking method>]_tractogram.json
+```
+
+Where "tract” is the anatomical/structural entity that is being imaged, and
+“track” uses as its value one of the items in a controlled vocabulary.
+
+For example:
+
+```Text
+
+AFQ/
+    sub–01/
+        dwi/
+            sub-01_space-MNI152NLin2009cAsym_tract-wholebrain_track-eudx_tractogram.trx
+            sub-01_space-MNI152NLin2009cAsym_tract-wholebrain_track-eudx_tractogram.json
+            sub-01_space-MNI152NLin2009cAsym_tract-ArcuateFasciculus_hemi-left_track-eudx_tractogram.trx
+            sub-01_space-MNI152NLin2009cAsym_tract-ArcuateFasciculus_hemi-left_track-eudx_tractogram.json
+```