diff --git a/src/main/java/org/gcube/informationsystem/base/reference/Direction.java b/src/main/java/org/gcube/informationsystem/base/reference/Direction.java
new file mode 100644
index 0000000..8b2c40f
--- /dev/null
+++ b/src/main/java/org/gcube/informationsystem/base/reference/Direction.java
@@ -0,0 +1,20 @@
+package org.gcube.informationsystem.base.reference;
+
+/**
+ * @author Luca Frosini (ISTI - CNR)
+ */
+public enum Direction {
+	
+	IN, OUT, BOTH;
+
+	public Direction opposite() {
+		if(this.equals(IN)) {
+			return OUT;
+		}else if(this.equals(OUT)) {
+			return IN;
+		} else {
+			return BOTH;
+		}
+	}
+	
+}