the number in Gtk.table

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from gi.repository import Gtk

class TableWindow(Gtk.Window):

    def __init__(self):

        Gtk.Window.__init__(self, title="Table Example")

        table = Gtk.Table(3, 3, True)

        self.add(table)

        button1 = Gtk.Button(label="Button 1")

        button2 = Gtk.Button(label="Button 2")

        button3 = Gtk.Button(label="Button 3")

        button4 = Gtk.Button(label="Button 4")

        button5 = Gtk.Button(label="Button 5")

        button6 = Gtk.Button(label="Button 6")

        table.attach(button1, 0, 1, 0, 1)

        table.attach(button2, 1, 3, 0, 1)

        table.attach(button3, 0, 1, 1, 3)

        table.attach(button4, 1, 3, 1, 2)

        table.attach(button5, 1, 2, 2, 3)

        table.attach(button6, 2, 3, 2, 3)

win = TableWindow()

win.connect("delete-event", Gtk.main_quit)

win.show_all()

Gtk.main()

when run the code ,i get  the attatchment,why   table.attach(button2, 1, 3, 0, 1)  ,what is the meaning of  1,3,0,1?