diff --git a/histogram_internal.h b/histogram_internal.h
new file mode 100644
index 0000000..b926f19
--- /dev/null
+++ b/histogram_internal.h
@@ -0,0 +1,6 @@
+#ifndef HISTOGRAM_INTERNAL_H_INCLUDED
+#define HISTOGRAM_INTERNAL_H_INCLUDED
+
+void find_minmax(const std::vector<double>& numbers, double &min, double &max);
+
+#endif // HISTOGRAM_INTERNAL_H_INCLUDED