diff --git a/src/sphinxnotes/any/indices.py b/src/sphinxnotes/any/indices.py
index 239233f..01d9e62 100644
--- a/src/sphinxnotes/any/indices.py
+++ b/src/sphinxnotes/any/indices.py
@@ -45,6 +45,10 @@ def derive(
             typ = f'Any{schema.objtype.title()}Index'
             name = schema.objtype
             localname = f'{schema.objtype.title()} Reference Index'
+        if classifier.by is not None:
+            typ += f'By{classifier.by.title()}'
+            name += '-by-' + classifier.by
+            localname += f'By {classifier.by.title()}'
         return type(
             typ,
             (cls,),
diff --git a/src/sphinxnotes/any/schema.py b/src/sphinxnotes/any/schema.py
index 59af8e9..ee11651 100644
--- a/src/sphinxnotes/any/schema.py
+++ b/src/sphinxnotes/any/schema.py
@@ -161,7 +161,7 @@ def __hash__(self):
 
 
 class Classifier(object):
-    name = ''
+    by: str | None = None
 
     @abstractmethod
     def classify(self, objref: Value) -> list[Classif]:
@@ -176,8 +176,6 @@ def sort(self, data: Iterable[_T], key: Callable[[_T], Classif]) -> list[_T]:
 
 
 class PlainClassifier(Classifier):
-    name = ''
-
     def classify(self, objref: Value) -> list[Classif]:
         entries = []
         for v in objref.as_list():
@@ -199,11 +197,17 @@ def sort(
 DISPFMTS_YM = '%Y 年 %m 月'
 DISPFMTS_MD = '%m 月 %d 日，%a'
 
+
 class YearClassifier(Classifier):
-    name = 'by-year'
+    by = 'year'
 
-    def __init__(self, inputfmts: list[str] = INPUTFMTS, dispfmt_y: str = DISPFMTS_Y,
-                 dispfmt_m: str = DISPFMTS_M, dispfmt_md: str = DISPFMTS_MD):
+    def __init__(
+        self,
+        inputfmts: list[str] = INPUTFMTS,
+        dispfmt_y: str = DISPFMTS_Y,
+        dispfmt_m: str = DISPFMTS_M,
+        dispfmt_md: str = DISPFMTS_MD,
+    ):
         """*xxxfmt* are date format used by time.strptime/strftime.
 
         .. seealso:: https://docs.python.org/3/library/datetime.html#strftime-and-strptime-format-codes"""
@@ -235,11 +239,16 @@ def sort(
         # TODO: Sort by time.
         return sorted(data, key=lambda x: key(x)._sort_key, reverse=True)
 
+
 class MonthClassifier(Classifier):
-    name = 'by-month'
+    by = 'month'
 
-    def __init__(self, inputfmts: list[str] = INPUTFMTS, dispfmt_ym: str = DISPFMTS_YM,
-                 dispfmt_md: str = DISPFMTS_MD):
+    def __init__(
+        self,
+        inputfmts: list[str] = INPUTFMTS,
+        dispfmt_ym: str = DISPFMTS_YM,
+        dispfmt_md: str = DISPFMTS_MD,
+    ):
         self.inputfmts = inputfmts
         self.dispfmt_ym = dispfmt_ym
         self.dispfmt_md = dispfmt_md
@@ -266,6 +275,7 @@ def sort(
         # TODO: Sort by time.
         return sorted(data, key=lambda x: key(x)._sort_key, reverse=True)
 
+
 @dataclasses.dataclass(frozen=True)
 class Object(object):
     objtype: str