diff --git a/beetsplug/lastgenre/__init__.py b/beetsplug/lastgenre/__init__.py
index 47f0f66ca..6f657fb48 100644
--- a/beetsplug/lastgenre/__init__.py
+++ b/beetsplug/lastgenre/__init__.py
@@ -41,6 +41,8 @@ if TYPE_CHECKING:
 
     from beets.library import LibModel
 
+    from .types import CanonTree
+
 
 class LastGenrePlugin(plugins.BeetsPlugin):
     def __init__(self) -> None:
@@ -116,7 +118,7 @@ class LastGenrePlugin(plugins.BeetsPlugin):
         return [p[1] for p in depth_tag_pairs]
 
     @staticmethod
-    def find_parents(candidate: str, branches: list[list[str]]) -> list[str]:
+    def find_parents(candidate: str, branches: CanonTree) -> list[str]:
         """Find parent genres of a given genre, ordered from closest to furthest."""
         for branch in branches:
             try:
diff --git a/beetsplug/lastgenre/client.py b/beetsplug/lastgenre/client.py
index 21a0bff72..bff472dc0 100644
--- a/beetsplug/lastgenre/client.py
+++ b/beetsplug/lastgenre/client.py
@@ -31,6 +31,8 @@ if TYPE_CHECKING:
 
     from beets.logging import Logger
 
+    from .types import GenreCache
+
 LASTFM = pylast.LastFMNetwork(api_key=plugins.LASTFM_KEY)
 
 PYLAST_EXCEPTIONS = (
@@ -51,7 +53,7 @@ class LastFmClient:
         self._log = log
         self._tunelog = make_tunelog(log)
         self._min_weight = min_weight
-        self._genre_cache: dict[str, list[str]] = {}
+        self._genre_cache: GenreCache = {}
 
     def fetch_genre(
         self, lastfm_obj: pylast.Album | pylast.Artist | pylast.Track
diff --git a/beetsplug/lastgenre/loaders.py b/beetsplug/lastgenre/loaders.py
index 0992ca14e..1f0720eeb 100644
--- a/beetsplug/lastgenre/loaders.py
+++ b/beetsplug/lastgenre/loaders.py
@@ -28,6 +28,8 @@ if TYPE_CHECKING:
 
     from beets.logging import Logger
 
+    from .types import CanonTree, Whitelist
+
 
 class DataFileLoader:
     """Loads genre-related data files for the lastgenre plugin."""
@@ -36,8 +38,8 @@ class DataFileLoader:
         self,
         log: Logger,
         plugin_dir: Path,
-        whitelist: set[str],
-        c14n_branches: list[list[str]],
+        whitelist: Whitelist,
+        c14n_branches: CanonTree,
         canonicalize: bool,
     ):
         """Initialize with pre-loaded data.
@@ -83,7 +85,7 @@ class DataFileLoader:
     @staticmethod
     def _load_whitelist(
         log: Logger, config_value: str | bool | None, default_path: str
-    ) -> set[str]:
+    ) -> Whitelist:
         """Load the whitelist from a text file.
 
         Returns set of valid genre names (lowercase).
@@ -107,12 +109,12 @@ class DataFileLoader:
         config_value: str | bool | None,
         default_path: str,
         prefer_specific: bool,
-    ) -> tuple[list[list[str]], bool]:
+    ) -> tuple[CanonTree, bool]:
         """Load the canonicalization tree from a YAML file.
 
         Returns tuple of (branches, canonicalize_enabled).
         """
-        c14n_branches: list[list[str]] = []
+        c14n_branches: CanonTree = []
         c14n_filename = config_value
         canonicalize = c14n_filename is not False
         # Default tree
@@ -133,9 +135,24 @@ class DataFileLoader:
     def flatten_tree(
         elem: dict[str, Any] | list[Any] | str,
         path: list[str],
-        branches: list[list[str]],
+        branches: CanonTree,
     ) -> None:
-        """Flatten nested lists/dictionaries into lists of strings (branches)."""
+        """Flatten nested YAML structure into genre hierarchy branches.
+
+        Recursively converts nested dicts/lists from YAML into a flat list
+        of genre paths, where each path goes from general to specific genre.
+
+        Args:
+            elem: The YAML element to process (dict, list, or string leaf).
+            path: Current path from root to this element (used in recursion).
+            branches: OUTPUT PARAMETER - Empty list that will be populated
+                     with genre paths. Gets mutated by this method.
+
+        Example:
+            branches = []
+            flatten_tree({'rock': ['indie', 'punk']}, [], branches)
+            # branches is now: [['rock', 'indie'], ['rock', 'punk']]
+        """
         if not path:
             path = []
 
diff --git a/beetsplug/lastgenre/types.py b/beetsplug/lastgenre/types.py
new file mode 100644
index 000000000..f8fa2aba5
--- /dev/null
+++ b/beetsplug/lastgenre/types.py
@@ -0,0 +1,30 @@
+# This file is part of beets.
+# Copyright 2026, J0J0 Todos.
+#
+# Permission is hereby granted, free of charge, to any person obtaining
+# a copy of this software and associated documentation files (the
+# "Software"), to deal in the Software without restriction, including
+# without limitation the rights to use, copy, modify, merge, publish,
+# distribute, sublicense, and/or sell copies of the Software, and to
+# permit persons to whom the Software is furnished to do so, subject to
+# the following conditions:
+#
+# The above copyright notice and this permission notice shall be
+# included in all copies or substantial portions of the Software.
+
+
+"""Type aliases for the lastgenre plugin."""
+
+from __future__ import annotations
+
+Whitelist = set[str]
+"""Set of valid genre names (lowercase). Empty set means all genres allowed."""
+
+CanonTree = list[list[str]]
+"""Genre hierarchy as list of paths from general to specific.
+Example: [['electronic', 'house'], ['electronic', 'techno']]"""
+
+GenreCache = dict[str, list[str]]
+"""Cache mapping entity keys to their genre lists.
+Keys are formatted as 'entity.arg1-arg2-...' (e.g., 'album.artist-title').
+Values are lists of lowercase genre strings."""