diff --git a/spacy/training/iob_utils.py b/spacy/training/iob_utils.py
index 03a502912..91fc40205 100644
--- a/spacy/training/iob_utils.py
+++ b/spacy/training/iob_utils.py
@@ -1,9 +1,11 @@
+from typing import List, Tuple, Iterable, Union, Iterator
 import warnings
+
 from ..errors import Errors, Warnings
-from ..tokens import Span
+from ..tokens import Span, Doc
 
 
-def iob_to_biluo(tags):
+def iob_to_biluo(tags: Iterable[str]) -> List[str]:
     out = []
     tags = list(tags)
     while tags:
@@ -12,7 +14,7 @@ def iob_to_biluo(tags):
     return out
 
 
-def biluo_to_iob(tags):
+def biluo_to_iob(tags: Iterable[str]) -> List[str]:
     out = []
     for tag in tags:
         if tag is None:
@@ -23,12 +25,12 @@ def biluo_to_iob(tags):
     return out
 
 
-def _consume_os(tags):
+def _consume_os(tags: List[str]) -> Iterator[str]:
     while tags and tags[0] == "O":
         yield tags.pop(0)
 
 
-def _consume_ent(tags):
+def _consume_ent(tags: List[str]) -> List[str]:
     if not tags:
         return []
     tag = tags.pop(0)
@@ -50,11 +52,7 @@ def _consume_ent(tags):
         return [start] + middle + [end]
 
 
-def biluo_tags_from_doc(doc, missing="O"):
-    return doc_to_biluo_tags(doc, missing)
-
-
-def doc_to_biluo_tags(doc, missing="O"):
+def doc_to_biluo_tags(doc: Doc, missing: str = "O"):
     return offsets_to_biluo_tags(
         doc,
         [(ent.start_char, ent.end_char, ent.label_) for ent in doc.ents],
@@ -62,11 +60,9 @@ def doc_to_biluo_tags(doc, missing="O"):
     )
 
 
-def biluo_tags_from_offsets(doc, entities, missing="O"):
-    return offsets_to_biluo_tags(doc, entities, missing)
-
-
-def offsets_to_biluo_tags(doc, entities, missing="O"):
+def offsets_to_biluo_tags(
+    doc: Doc, entities: Iterable[Tuple[int, int, Union[str, int]]], missing: str = "O"
+) -> List[str]:
     """Encode labelled spans into per-token tags, using the
     Begin/In/Last/Unit/Out scheme (BILUO).
 
@@ -77,7 +73,7 @@ def offsets_to_biluo_tags(doc, entities, missing="O"):
         the original string.
     RETURNS (list): A list of unicode strings, describing the tags. Each tag
         string will be of the form either "", "O" or "{action}-{label}", where
-        action is one of "B", "I", "L", "U". The string "-" is used where the
+        action is one of "B", "I", "L", "U". The missing label is used where the
         entity offsets don't align with the tokenization in the `Doc` object.
         The training algorithm will view these as missing values. "O" denotes a
         non-entity token. "B" denotes the beginning of a multi-token entity,
@@ -93,7 +89,6 @@ def offsets_to_biluo_tags(doc, entities, missing="O"):
     """
     # Ensure no overlapping entity labels exist
     tokens_in_ents = {}
-
     starts = {token.idx: token.i for token in doc}
     ends = {token.idx + len(token): token.i for token in doc}
     biluo = ["-" for _ in doc]
@@ -117,7 +112,6 @@ def offsets_to_biluo_tags(doc, entities, missing="O"):
                         )
                     )
                 tokens_in_ents[token_index] = (start_char, end_char, label)
-
             start_token = starts.get(start_char)
             end_token = ends.get(end_char)
             # Only interested if the tokenization is correct
@@ -151,11 +145,7 @@ def offsets_to_biluo_tags(doc, entities, missing="O"):
     return biluo
 
 
-def spans_from_biluo_tags(doc, tags):
-    return biluo_tags_to_spans(doc, tags)
-
-
-def biluo_tags_to_spans(doc, tags):
+def biluo_tags_to_spans(doc: Doc, tags: Iterable[str]) -> List[Span]:
     """Encode per-token tags following the BILUO scheme into Span object, e.g.
     to overwrite the doc.ents.
 
@@ -173,11 +163,9 @@ def biluo_tags_to_spans(doc, tags):
     return spans
 
 
-def offsets_from_biluo_tags(doc, tags):
-    return biluo_tags_to_offsets(doc, tags)
-
-
-def biluo_tags_to_offsets(doc, tags):
+def biluo_tags_to_offsets(
+    doc: Doc, tags: Iterable[str]
+) -> List[Tuple[int, int, Union[str, int]]]:
     """Encode per-token tags following the BILUO scheme into entity offsets.
 
     doc (Doc): The document that the BILUO tags refer to.
@@ -192,8 +180,8 @@ def biluo_tags_to_offsets(doc, tags):
     return [(span.start_char, span.end_char, span.label_) for span in spans]
 
 
-def tags_to_entities(tags):
-    """ Note that the end index returned by this function is inclusive.
+def tags_to_entities(tags: Iterable[str]) -> List[Tuple[str, int, int]]:
+    """Note that the end index returned by this function is inclusive.
     To use it for Span creation, increment the end by 1."""
     entities = []
     start = None
@@ -225,3 +213,9 @@ def tags_to_entities(tags):
         else:
             raise ValueError(Errors.E068.format(tag=tag))
     return entities
+
+
+# Fallbacks to make backwards-compat easier
+offsets_from_biluo_tags = biluo_tags_to_offsets
+spans_from_biluo_tags = biluo_tags_to_spans
+biluo_tags_from_offsets = offsets_to_biluo_tags