During whaling times, the custom of fashioning a small round button from a section of Sperm Whale tooth and having it mounted to the top of the newel post of your home became popular.
The presence of a Newel Post Button told visitors that there were no liens on the property. Hence, over time, the name changed to Mortgage Button.
I ordered one a month ago as we paid off the house.
It is installed now and while very few visitors go there or will even understand it, all that matters is that we know what it is and what it means to us, so we can smile every time we go by.