This 3-ring adapter is used to increase the number of works that a choral folder can hold. It's basically the center part of a traditional 3-ring binder you'd find in an office or classroom. Nominally 1/2-inch, the rings have a 3/4-inch outside diameter.Choral folders that hold music in the center are limited in how many chorals they can hold by the number of strings (e.g. five strings means a maximum of five chorals). This adapter allows you to add a number of additional chorals (3-hole punched) along with those being held by the strings. The user removes one of the strings and threads it through the mounting holes in the 3-ring attachment, then the additional chorals are clamped into the attachment. It's a clever and effective workaround. View the video below to see how this is done, as it is hard to explain it here. -cdp