I am often asked "What is a Whistle Button"
It is a term strictly used by collectors, it
refers to all buttons constructed with one hole on the top and two or
more holes on the back. This was done to save on the wear and tear of
the thread. When the button is sewn on, the thread becomes recessed,
therefore protecting it from daily wear. Whistle buttons can be found
in a wide variety of materials.
Henriettas Vintage - Whistle Buttons