diff --git a/README.md b/README.md
index 1ab1ff7..c01ef1a 100644
--- a/README.md
+++ b/README.md
@@ -12,7 +12,7 @@ Ends with `.mini` i.e. `config.mini`.
 - All (sub-) sections must be explicitly defined before use.
 - (Sub-) sections can be empty.
 - Keys within sections or subsections use `key = value` syntax.
-- Key- and section-names may only contain `a-z`, `A-Z`, and `0-9`.
+- Key- and section-names may only contain `a-z`, `A-Z`, `0-9`, and `_`.
 - Empty keys (i.e., keys without values) are not allowed.
 - Spaces outside of strings are ignored.
 - Comments are denoted by `#` and may only appear on separate lines.