[Tutor] What does parent=0 mean?

[Klas Marteleur]

Hi
I am going thru theGUI programing chapter in Alans latest web tutorial. And 
found this example:

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from Tkinter import *
     
class ClearApp:
   def __init__(self, parent=0):
      self.mainWindow = Frame(parent)
      # Create the entry widget
      self.entry = Entry(self.mainWindow)
      self.entry.insert(0,"Hello world")
      self.entry.pack(fill=X)
      
      # now add the 2 buttons, use a grooved effect 
      fButtons = Frame(fOuter, border=2, relief="groove")
      bClear = Button(fButtons, text="Clear", 
                      width=8, height=1, command=self.clearText)
      bQuit = Button(fButtons, text="Quit", 
                      width=8, height=1, command=self.mainWindow.quit)
      bClear.pack(side="left", padx=15, pady=1)
      bQuit.pack(side="right", padx=15, pady=1)
      fButtons.pack(fill=X)
      self.mainWindow.pack()

      # set the title
      self.mainWindow.master.title("Clear")
      
   def clearText(self):
      self.entry.delete(0,END)
      
app = ClearApp()
app.mainWindow.mainloop()
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What does parent=0 mean?

Klas