The wx.ComboBox is a combination of an editbox and a (dropdown) listbox. It let's the user slect an item from the listbox and puts it in the editbox. In this snippet we use two comboboxes, each used to select an area unit of measurement. Once selected, an area value can be converted from a unit like acres to another unit, for instance squaremiles. Additional wxPython widgets like wx.Frame, wx.Panel, wx.ToolTip, wx.StaticText, wx.TextCtrl, and wx.Button are part of this demonstration. The layout is done entirely with upper left corner position (x, y) tuples and size (width, height) tuples. This demo is supposed to illustrate the layout too, so I used plenty of key-words like pos=wx.Point(x, y) and size=wx.Size(width, height).