diff --git a/scripts/extract.py b/scripts/extract.py
index efc1e0d..a8b584b 100644
--- a/scripts/extract.py
+++ b/scripts/extract.py
@@ -1,4 +1,12 @@
-"""Extract script and region constants from Script.h."""
+"""Extract various constants from Mac OS header files.
+
+This program reads Script.h and TextCommon.h from Macintosh headers. Pass the
+paths to one or both of these files when invoking this program, and the program
+will write out the corresponding CSV files.
+
+It does not matter what line endings or character encodings the headers use, as
+long as the encoding is ASCII-compatible.
+"""
 import csv
 import os
 import re
@@ -9,7 +17,7 @@ from typing import Iterator, List, Tuple
 Item = Tuple[str, int]
 
 def list_enums(filename: str) -> Iterator[Item]:
-    """List enum definitions in a file."""
+    """List enum definitions in a header file."""
     with open(filename, 'rb') as fp:
         data = fp.read()
     for item in re.finditer(
@@ -19,15 +27,18 @@ def list_enums(filename: str) -> Iterator[Item]:
         yield name.decode('ASCII'), int(value, 0)
 
 def index_of(data: List[Item], key: str) -> int:
+    """Return the index of the enum with the given name."""
     for i, (name, _) in enumerate(data):
         if name == key:
             return i
     raise ValueError('missing value: {!r}'.format(key))
 
 def slice(data: List[Item], first: str, last: str) -> List[Item]:
+    """Return a slice of enums, by giving the name of the first and last."""
     return data[index_of(data, first):index_of(data, last)+1]
 
 def write_csv(fname: str, data: List[Item]) -> None:
+    """Write a CSV file containing enum values."""
     print('Writing', fname, file=sys.stderr)
     with open(fname, 'w') as fp:
         w = csv.writer(fp)
@@ -36,6 +47,7 @@ def write_csv(fname: str, data: List[Item]) -> None:
             w.writerow(item)
 
 def process_script(filename: str) -> None:
+    """Process the <Script.h> header file."""
     scripts: List[Item] = []
     regions: List[Item] = []
     for name, value in list_enums(filename):
@@ -47,6 +59,7 @@ def process_script(filename: str) -> None:
     write_csv('region.csv', slice(regions, 'verUS', 'verGreenland'))
 
 def process_textcommon(filename: str) -> None:
+    """Process the <TextCommon.h> header file."""
     encodings: List[Item] = []
     for name, value in list_enums(filename):
         if name.startswith('kTextEncoding'):
@@ -54,6 +67,7 @@ def process_textcommon(filename: str) -> None:
     write_csv('encoding.csv', encodings)
 
 def process(filename: str) -> None:
+    """Process any header file."""
     name = os.path.basename(filename).lower()
     if name == 'script.h':
         process_script(filename)