On some shirts, any or all of the following may be true of the lowermost button/buttonhole:
The buttonhole is oriented horizontally (whereas all the others are oriented vertically)
The buttonhole is lined with a different colour of thread from the others
The button is sewn on with a different colour of thread from the others
I’m not talking about spare buttons sewn onto the bottom corner of the shirt - this is a proper button/buttonhole, it’s just different from all the others - why?