diff --git a/src/sctools/bam.py b/src/sctools/bam.py
index 77040de..ea3bddc 100644
--- a/src/sctools/bam.py
+++ b/src/sctools/bam.py
@@ -448,6 +448,7 @@ def key_generator(self) -> Callable[[pysam.AlignedSegment], Any]:
 
 
 class QueryNameSortOrder(AlignmentSortOrder):
+    """Alignment record sort order by query name."""
     @staticmethod
     def get_sort_key(alignment: pysam.AlignedSegment) -> str:
         return alignment.query_name
@@ -461,6 +462,7 @@ def __repr__(self) -> str:
 
 
 class CellMoleculeGeneQueryNameSortOrder(AlignmentSortOrder):
+    """Hierarchical alignment record sort order (cell barcode >= molecule barcode >= gene name >= query name)."""
     def __init__(
             self,
             cell_barcode_tag_key: str = consts.CELL_BARCODE_TAG_KEY,
@@ -473,7 +475,7 @@ def __init__(
         self.molecule_barcode_tag_key = molecule_barcode_tag_key
         self.gene_name_tag_key = gene_name_tag_key
 
-    def get_sort_key(self, alignment: pysam.AlignedSegment) -> Tuple[str, str, str, str]:
+    def _get_sort_key(self, alignment: pysam.AlignedSegment) -> Tuple[str, str, str, str]:
         return (get_tag_or_default(alignment, self.cell_barcode_tag_key),
                 get_tag_or_default(alignment, self.molecule_barcode_tag_key),
                 get_tag_or_default(alignment, self.gene_name_tag_key),
@@ -481,7 +483,7 @@ def get_sort_key(self, alignment: pysam.AlignedSegment) -> Tuple[str, str, str,
 
     @property
     def key_generator(self) -> Callable[[pysam.AlignedSegment], Tuple[str, str, str, str]]:
-        return self.get_sort_key
+        return self._get_sort_key
 
     def __repr__(self) -> str:
         return 'hierarchical__cell_molecule_gene_query_name'