diff --git a/src/main/java/eu/dnetlib/dhp/schema/common/ModelConstants.java b/src/main/java/eu/dnetlib/dhp/schema/common/ModelConstants.java
index 5c08765..ed9bf48 100644
--- a/src/main/java/eu/dnetlib/dhp/schema/common/ModelConstants.java
+++ b/src/main/java/eu/dnetlib/dhp/schema/common/ModelConstants.java
@@ -148,6 +148,10 @@ public class ModelConstants {
 
 	public static final String IS_DIFFERENT_FROM = "isDifferentFrom";
 
+	public static final String HIERARCHY = "hierarchy"; // subreltype
+	public static final String IS_PARENT_OF = "isParentOf";
+	public static final String IS_CHILD_OF = "isChildOf";
+
 	public static final String UNKNOWN = "UNKNOWN";
 	public static final String NOT_AVAILABLE = "not available";