diff --git a/history/005_letter_test/src/main/java/com/github/drinkjava2/frog/brain/Relation.java b/history/005_letter_test/src/main/java/com/github/drinkjava2/frog/brain/Relation.java
new file mode 100644
index 0000000..c6a7d2f
--- /dev/null
+++ b/history/005_letter_test/src/main/java/com/github/drinkjava2/frog/brain/Relation.java
@@ -0,0 +1,31 @@
+/*
+ * Copyright 2018 the original author or authors. 
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by
+ * applicable law or agreed to in writing, software distributed under the
+ * License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS
+ * OF ANY KIND, either express or implied. See the License for the specific
+ * language governing permissions and limitations under the License.
+ */
+package com.github.drinkjava2.frog.brain;
+
+/**
+ * Relation is the relationship betweeen 2 holes
+ * 
+ * Relation 表示细胞上的两个洞之间存在关联关系，多对多关系用很多个一对一关系来表达
+ * 
+ * @author Yong Zhu
+ * @since 2.0.2
+ */
+public class Relation {
+	public Hole h1;
+	public Hole h2;
+	public int strength = 1;
+
+	public Relation(Hole h1, Hole h2) {
+		this.h1 = h1;
+		this.h2 = h2;
+	}
+
+}