diff --git a/05-trees/BinaryTree.cpp b/05-trees/BinaryTree.cpp
index bc5934c..3275a0f 100644
--- a/05-trees/BinaryTree.cpp
+++ b/05-trees/BinaryTree.cpp
@@ -18,61 +18,68 @@ BinaryTree::BinaryTree() {
 
 /**~*~*
    This function calls a recursive function to traverse the
-   tree in postorder (the wrapper function)
+   tree in postorder
 *~**/
-void BinaryTree::postOrder() const {
-    _postOrder(root);
+void BinaryTree::postOrder(void visit(const Data &)) const {
+    _postOrder(root, visit);
 }
 
 /**~*~*
    Postorder Traversal of the Binary Tree:
-   Left-Right-Root (recursive)
+   Left-Right-Root
 *~**/
-void BinaryTree::_postOrder(BinaryTree::Node *root) const {
+void BinaryTree::_postOrder(BinaryTree::Node *root, void visit(const Data &)) const {
     if (root == nullptr) return;
 
-    _postOrder(root->left);
-    _postOrder(root->right);
-    cout << root->data.num << " ";
+    _postOrder(root->left, visit);
+    _postOrder(root->right, visit);
+    visit(root->data);
 }
 
 /**~*~*
    This function calls a recursive function to traverse the
-   tree in preorder (the wrapper function)
+   tree in preorder
 *~**/
-void BinaryTree::preOrder() const {
-    _preOrder(root);
+void BinaryTree::preOrder(void visit(const Data &)) const {
+    _preOrder(root, visit);
 }
 
 /**~*~*
-   Preorder Traversal of the Binary Tree:
-   Root-Left-Right (recursive)
+   Postorder Traversal of the Binary Tree:
+   Left-Right-Root
 *~**/
-void BinaryTree::_preOrder(BinaryTree::Node *root) const {
+void BinaryTree::_preOrder(BinaryTree::Node *root, void visit(const Data &)) const {
     if (root == nullptr) return;
 
-    cout << root->data.num << " ";
-    _preOrder(root->left);
-    _preOrder(root->right);
+    visit(root->data);
+    _preOrder(root->left, visit);
+    _preOrder(root->right, visit);
 }
 
 /**~*~*
    This function calls a recursive function to traverse the
-   tree in inorder (the wrapper function)
+   tree in inorder
 *~**/
-void BinaryTree::inOrder() const {
-    _inOrder(root);
+void BinaryTree::inOrder(void visit(const Data &)) const {
+    _inOrder(root, visit);
 }
 
 /**~*~*
    Inorder Traversal of the Binary Tree:
-   Left-Root-Right (recursive)
+   Left-Root-Right
 *~**/
-void BinaryTree::_inOrder(Node *root) const {
+void BinaryTree::_inOrder(Node *root, void visit(const Data &)) const {
     if (root) {
-        _inOrder(root->left);
-        cout << root->data.num << " ";
-        _inOrder(root->right);
+        _inOrder(root->left, visit);
+        //cout << root->data.num << " ";
+        visit(root->data);
+        // cout has been replaced with a call for visit
+        // What is visit?
+        // visit is a generic name for a display function
+        // in main(), when inOrder is called, it is decided what function address to assign to visit
+        // here, you just use visit the way you would use/call a function
+        _inOrder(root->right, visit);
+        // ------------------^^^^^^ visit as an argument
     }
 }
 
diff --git a/05-trees/BinaryTree.h b/05-trees/BinaryTree.h
index 40ef39e..1d7520d 100644
--- a/05-trees/BinaryTree.h
+++ b/05-trees/BinaryTree.h
@@ -28,18 +28,18 @@ public:
     // Binary Tree operations
     void insert(Data dataIn);
 
-    void inOrder() const;
+    void inOrder(void visit(const Data &)) const;
 
-    void preOrder() const;
+    void preOrder(void visit(const Data &)) const;
 
-    void postOrder() const;
+    void postOrder(void visit(const Data &)) const;
 
 private:
-    void _inOrder(Node *root) const;
+    void _inOrder(Node *root, void visit(const Data &)) const;
 
-    void _preOrder(Node *root) const;
+    void _preOrder(Node *root, void visit(const Data &)) const;
 
-    void _postOrder(Node *root) const;
+    void _postOrder(Node *root, void visit(const Data &)) const;
 
     void _destroy(Node *root);
 };
diff --git a/05-trees/BinaryTree_Demo.cpp b/05-trees/BinaryTree_Demo.cpp
index e02359e..75f0527 100644
--- a/05-trees/BinaryTree_Demo.cpp
+++ b/05-trees/BinaryTree_Demo.cpp
@@ -22,6 +22,9 @@ using namespace std;
 
 void build_BT(BinaryTree &tree, int n);
 
+void hDisplay(const Data &item); // horizontal display: all items on one line
+void vDisplay(const Data &item); // vertical display: one item per line
+
 int main(void) {
     BinaryTree tree;
     int n; // number of nodes
@@ -34,17 +37,21 @@ int main(void) {
     build_BT(tree, n);
 
     cout << "  Inorder: ";
-    tree.inOrder();
+    tree.inOrder(hDisplay); // hDisplay is the inOrder's argument
     cout << endl;
 
     if (option == 'T' || option == 't') {
         cout << "Postorder: ";
-        tree.postOrder();
+        tree.postOrder(hDisplay); // passing hDisplay to postOrder
+        cout << endl;
+        tree.postOrder(vDisplay); // passing vDisplay to postOrder
         cout << endl;
     }
     if (option == 'E' || option == 'e') {
         cout << " Preorder: ";
-        tree.preOrder();
+        tree.preOrder(hDisplay);  // passing hDisplay to preOrder
+        cout << endl;
+        tree.preOrder(vDisplay);  // passing vDisplay to preOrder
         cout << endl;
     }
     return 0;
@@ -66,3 +73,18 @@ void build_BT(BinaryTree &tree, int n) {
         tree.insert(data);
     }
 }
+
+// The following two functions are used as arguments to other functions
+/**~*~*
+   horizontal display: all items on one line
+*/
+void hDisplay(const Data &item) {
+    cout << item.num << " ";
+}
+
+/**~*~*
+   // vertical display: one item per line
+*/
+void vDisplay(const Data &item) {
+    cout << item.num << endl;
+}