diff --git a/tools/schemacode/bidsschematools/render.py b/tools/schemacode/bidsschematools/render.py
index 7af32cc3dc..1ee8e7cb95 100644
--- a/tools/schemacode/bidsschematools/render.py
+++ b/tools/schemacode/bidsschematools/render.py
@@ -7,6 +7,7 @@
 
 import pandas as pd
 import yaml
+from markdown_it import MarkdownIt
 from tabulate import tabulate
 
 from . import utils
@@ -469,11 +470,43 @@ def make_filename_template(
         )
 
     codeblock = codeblock.expandtabs(4)
+    codeblock = append_filename_template_legend(codeblock, pdf_format)
     codeblock = codeblock.replace("SPEC_ROOT", get_relpath(src_path))
 
     return codeblock
 
 
+def append_filename_template_legend(text, pdf_format=False):
+    legend = """
+- Filename entities or folders between square brackets
+  (for example, `[_ses-<label>]`) are OPTIONAL.
+- Some entities may only allow specific values,
+  in which case those values are listed in `<>`, separated by `|`.
+- `_<suffix>` means that there are several (>6) valid suffixes for this filename pattern.
+- `.<extension>` means that there are several (>6) valid extensions for this file type.
+- `[.gz]` means that both the unzipped and gzipped versions of the extension are valid.
+"""
+
+    if pdf_format:
+        text += f"""
+
+**Legend**:
+
+{legend}
+
+"""
+    else:
+        md = MarkdownIt()
+        text += f"""
+<details>
+<summary><strong>Legend:</strong></summary>
+{md.render(legend)}
+</details>
+"""
+
+    return text
+
+
 def make_entity_table(schema, tablefmt="github", **kwargs):
     """Produce entity table (markdown) based on schema.
 
diff --git a/tools/schemacode/setup.cfg b/tools/schemacode/setup.cfg
index eb55393ddd..5968790370 100644
--- a/tools/schemacode/setup.cfg
+++ b/tools/schemacode/setup.cfg
@@ -34,6 +34,7 @@ doc =
 render =
     tabulate
     pandas
+    markdown-it-py
 tests =
     codecov
     coverage[toml]