diff --git a/Histogram/histogram_internal.h b/Histogram/histogram_internal.h
new file mode 100644
index 0000000..65e1d47
--- /dev/null
+++ b/Histogram/histogram_internal.h
@@ -0,0 +1,3 @@
+#pragma once
+#include <vector>
+size_t find_minmax(const std::vector<double>& numbers, double& min, double& max);