diff --git a/file/3Ddisplay.md b/file/3Ddisplay.md
index 9f662c8..dbd676e 100644
--- a/file/3Ddisplay.md
+++ b/file/3Ddisplay.md
@@ -1,2 +1,8 @@
 ### 3Ddisplay
 display something in the terminal in the form of 3D.
+### Principle
++ graph-described file
++ picture-displayed file 
++ a engine to process the file and output ascii pictures-displayed file
++ a converter to tranform pictures-displayed file into something displayed in the terminal
+