diff --git a/.editorconfig b/.editorconfig
index a00a45c..4cfaf7d 100644
--- a/.editorconfig
+++ b/.editorconfig
@@ -8,7 +8,9 @@ trim_trailing_whitespace = true
 
 [*.md]
 trim_trailing_whitespace = false
+max_line_length=79
 
 [*.py]
 indent_style = tab
 indent_size = 4
+max_line_length=79
diff --git a/doc/types.md b/doc/types.md
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+++ b/doc/types.md
@@ -0,0 +1,14 @@
+# Special types used by cppo
+
+- *hex-ustr*
+  This is a standard Python3 string (UTF-8) containing two hex digits per byte.
+  whose length should always be an even number.  No prefix or suffix is used,
+  just the digits.  It's just a convenient way to specify an arbitrary stream
+  of bytes in a human-readable format.  This is somewhat inefficient as
+  implemented because the functions are designed to work in Python2 and
+  Python3.
+
+- *bin-ustr*
+  Like hex-ustr but composed of only the digits 0 and 1.  You need eight of
+  these to make a byte.  As with hex-ustr, functions operating on bin-ustr jump
+  through hoops to be compatible with both Python2 and Python3.