diff --git a/histogram_internal.h b/histogram_internal.h
new file mode 100644
index 0000000..34ffb8d
--- /dev/null
+++ b/histogram_internal.h
@@ -0,0 +1,6 @@
+#pragma once
+#include <vector>
+
+void find_minmax(const std::vector<double>& numbers,
+    double& min,
+    double& max);