diff --git a/core/prioritize.py b/core/prioritize.py
index b5be1a46..265cb284 100644
--- a/core/prioritize.py
+++ b/core/prioritize.py
@@ -24,19 +24,11 @@ class CriterionCategory:
     def format_criterion_value(self, value):
         return value
     
-    #--- Public
     def sort_key(self, dupe, crit_value):
-        # Use this sort key when the order in the list depends on whether or not the dupe meets the
-        # criteria. If it does, we return 0 (top of the list), if it doesn't, we return 1.
-        if self.extract_value(dupe) == crit_value:
-            return 0
-        else:
-            return 1
+        raise NotImplementedError()
     
     def criteria_list(self):
-        dupes = flatten(g[:] for g in self.results.groups)
-        values = dedupe(self.extract_value(d) for d in dupes)
-        return [Criterion(self, value) for value in values]
+        raise NotImplementedError()
 
 class Criterion:
     def __init__(self, category, value):
@@ -52,13 +44,31 @@ class Criterion:
         return "{} ({})".format(self.category.NAME, self.display_value)
     
 
-class KindCategory(CriterionCategory):
+class ValueListCategory(CriterionCategory):
+    def sort_key(self, dupe, crit_value):
+        # Use this sort key when the order in the list depends on whether or not the dupe meets the
+        # criteria. If it does, we return 0 (top of the list), if it doesn't, we return 1.
+        if self.extract_value(dupe) == crit_value:
+            return 0
+        else:
+            return 1
+    
+    def criteria_list(self):
+        dupes = flatten(g[:] for g in self.results.groups)
+        values = sorted(dedupe(self.extract_value(d) for d in dupes))
+        return [Criterion(self, value) for value in values]
+    
+
+class KindCategory(ValueListCategory):
     NAME = coltr("Kind")
     
     def extract_value(self, dupe):
-        return dupe.extension
+        value = dupe.extension
+        if not value:
+            value = tr("None")
+        return value
 
-class FolderCategory(CriterionCategory):
+class FolderCategory(ValueListCategory):
     NAME = coltr("Folder")
     
     def extract_value(self, dupe):
@@ -66,6 +76,13 @@ class FolderCategory(CriterionCategory):
     
     def format_criterion_value(self, value):
         return str(value)
+    
+    def sort_key(self, dupe, crit_value):
+        value = self.extract_value(dupe)
+        if value[:len(crit_value)] == crit_value:
+            return 0
+        else:
+            return 1
 
 class FilenameCategory(CriterionCategory):
     NAME = coltr("Filename")
diff --git a/core/tests/prioritize_test.py b/core/tests/prioritize_test.py
index b421c36e..bfa4b64d 100644
--- a/core/tests/prioritize_test.py
+++ b/core/tests/prioritize_test.py
@@ -137,3 +137,39 @@ def test_filename_reprioritization(app):
     app.add_pri_criterion("Filename", 0) # Ends with a number
     app.pdialog.perform_reprioritization()
     eq_(app.rtable[0].data[0], 'foo1.ext')
+
+#---
+def app_with_subfolders():
+    dupes = [
+        [
+            no('foo1', folder='baz'),
+            no('foo2', folder='foo/bar'),
+        ],
+        [
+            no('foo3', folder='baz'),
+            no('foo4', folder='foo'),
+        ],
+    ]
+    return app_with_dupes(dupes)
+
+@with_app(app_with_subfolders)
+def test_folder_crit_is_sorted(app):
+    # Folder subcriteria are sorted.
+    app.select_pri_criterion("Folder")
+    eq_(app.pdialog.criteria_list[:], ['baz', 'foo', 'foo/bar'])
+
+@with_app(app_with_subfolders)
+def test_folder_crit_includes_subfolders(app):
+    # When selecting a folder crit, dupes in a subfolder are also considered as affected by that
+    # crit.
+    app.add_pri_criterion("Folder", 1) # foo
+    app.pdialog.perform_reprioritization()
+    # Both foo and foo/bar dupes will be prioritized
+    eq_(app.rtable[0].data[0], 'foo2')
+    eq_(app.rtable[2].data[0], 'foo4')
+
+@with_app(app_with_subfolders)
+def test_display_something_on_empty_extensions(app):
+    # When there's no extension, display "None" instead of nothing at all.
+    app.select_pri_criterion("Kind")
+    eq_(app.pdialog.criteria_list[:], ['None'])