Userforms are Application objects, just like ThisWorkbook, Sheet1, Sheet2, Module1, etc. When you execute the UserForm.Show command, the application looks for that object in the Pruject list. If you open the vb editor (Alt + F11) you can see the Project list in the upper left corner of the editor window. If a userform of the name specified exists in the Project list, it will become visible on the screen. You can get more detailed information by doing a web search on 'Excel UserForms'.