Buttons without holes in their surface have a built-in button shank. This shank is a loop on the back of the button through which thread is sewn in order to attach the button to a garment.
The shank can be molded onto the back of the button during production. Alternatively, it can be constructed as a separate piece and later fixed onto the button back.
There are a number of different built-in button shank types. Here are a few:
Tunnel shank.
Tunnel shank buttons are so-called because the shank seems to tunnel through the underside of the button. These buttons attach very closely to a garment, which makes them perfect for very fine fabrics and garment embellishment.
Round, metal shank.
Round, metal shanks are usually made from copper, which can then be attached to buttons made from ABS or acrylic plastic. These button shanks allow for more space between the button and the garment, perfect for firmly attached, functional fastenings.
Square shank.
In this example we have a square shank that has been molded from plastic at the same time as the rest of the button. This is another button shank that can be firmly attached and that works well for fastening garments.