diff --git a/redstring/library.py b/redstring/library.py
index d74d325..a3eecbb 100644
--- a/redstring/library.py
+++ b/redstring/library.py
@@ -3,7 +3,15 @@ Logic for managing documents.
 """
 import os
 
-from redstring.parser import load, DocumentTag, DocumentTab, Document, TabOptions, TagOptions
+from redstring.parser import (
+    Document,
+    DocumentTab,
+    DocumentTag,
+    DocumentSubtag,
+    load,
+    TabOptions,
+    TagOptions,
+)
 
 
 def generate_index_document(directory: str) -> Document:
@@ -12,31 +20,48 @@ def generate_index_document(directory: str) -> Document:
     """
     categories: dict = {}
 
+    # categories = {
+    #   tab_name: {
+    #     tag_name: {
+    #       'documents': [ (id, title), ... ]
+    #       'subtags': {
+    #         subtag_name: [ (id, title), ... ]
+    #       }
+    #     }
+    #   }
+    # }
+
     for filename in os.listdir(directory):
         with open(os.path.join(directory, filename)) as f:
             document: Document = load(f)
 
         # Check if this document specifies a tab, and create it if necessary.
         if category_tag := document.get_tag('category'):
-            category = category_tag.value
+            category_name = category_tag.value
         else:
-            category = 'index'
-        if category not in categories:
-            categories[category] = {}
-        category_tab = categories[category]
+            category_name = 'index'
+        if category_name not in categories:
+            categories[category_name] = {}
+        destination_category = categories[category_name]
 
         # Check if this document specifies a topic, and create it if necessary.
         if topic_tag := document.get_tag('topic'):
-            topic = topic_tag.value
+            topic_name, subtopic_name = topic_tag.value, None
         else:
-            topic = 'uncategorized'
-        if '.' in topic:
-            topic, subtopic = topic.split('.', maxsplit=1)
+            topic_name, subtopic_name = 'uncategorized', None
+        if '.' in topic_name:
+            topic_name, subtopic_name = topic_name.split('.', maxsplit=1)
+        if topic_name not in destination_category:
+            destination_category[topic_name] = {
+                'documents': [],
+                'subtopics': {},
+            }
+        if subtopic_name:
+            if subtopic_name not in destination_category[topic_name]['subtopics']:
+                destination_category[topic_name]['subtopics'][subtopic_name] = []
+            destination_topic = destination_category[topic_name]['subtopics'][subtopic_name]
         else:
-            subtopic = None
-        if topic not in category_tab:
-            category_tab[topic] = []
-        topic_tag = category_tab[topic]
+            destination_topic = destination_category[topic_name]['documents']
 
         # Save the title and id.
         doc_id = document.get_tag('id').value
@@ -45,7 +70,7 @@ def generate_index_document(directory: str) -> Document:
         else:
             doc_title = None
 
-        topic_tag.append((doc_id, doc_title))
+        destination_topic.append((doc_id, doc_title))
 
     # Build an index document
     def document_link(info):
@@ -57,16 +82,24 @@ def generate_index_document(directory: str) -> Document:
         )
 
     built_tabs: list = []
-    for category in sorted(categories.keys()):
+    for category_name, category in sorted(categories.items()):
+
         built_tags: list = []
-
-        for topic in sorted(categories[category].keys()):
-            docs = sorted(categories[category][topic], key=lambda x: x[0])
+        for topic_name, topic in sorted(category.items()):
+            docs = sorted(topic['documents'], key=lambda x: x[0])
             doc_links = map(document_link, docs)
-            value = '- ' + '<br>- '.join(doc_links)
-            built_tags.append(DocumentTag(topic, value))
+            value = '- ' + '<br>- '.join(doc_links) if docs else ''
 
-        built_tabs.append(DocumentTab(category, built_tags))
+            built_subtags: list = []
+            for subtopic_name, subtopic in sorted(topic['subtopics'].items()):
+                docs = sorted(subtopic, key = lambda x: x[0])
+                doc_links = map(document_link, docs)
+                subtag_value = '- ' + '<br>- '.join(doc_links) if docs else ''
+                built_subtags.append(DocumentSubtag(subtopic_name, subtag_value))
+
+            built_tags.append(DocumentTag(topic_name, value, subtags=built_subtags))
+
+        built_tabs.append(DocumentTab(category_name, built_tags))
 
     return Document(built_tabs)