This is an invisible tie fastener. Just button it onto the existing buttons of your shirt and it slips through the keeper loop on the back of the front tie blade. Keeps everything nice and tighty with a clean look.
I printed it in Nijaflex, and would recommend printing in some kind of flexible filament.
There are two stls, one of them is 2mm thick, the other is a little slimmer at 1mm thick for a more flexible tie buddy. But both of them are pretty flexible.
Depending on the spacing of the buttons on your shirt this may not fit all shirts, however it seems to work on all the shirts I have, even though they are from a variety of brands, the spacing must be standardized.
Sorry the pictures kinda suck.