diff --git a/histogram.iternal.h b/histogram.iternal.h
new file mode 100644
index 0000000..0698c96
--- /dev/null
+++ b/histogram.iternal.h
@@ -0,0 +1,8 @@
+#ifndef HISTOGRAM_INTERNAL_H_INCLUDED
+#define HISTOGRAM_INTERNAL_H_INCLUDED
+
+#include <vector>
+
+bool find_minmax(const std::vector<double>& numbers, double& min, double& max);
+
+#endif