Tie Bar

A tie clip (also tie slide, tie bar, or tie clasp) is a clothing accessory that is used to clip a tie to the underlying shirt front, preventing it from swinging and ensuring that the tie hangs straight, resulting in a neat, uniform appearance.

When not to wear a tie bar?
Since the primary function of the tie bar is to hold the tie in place, you should only wear one when it's needed. If you are wearing a tie under a sweater or buttoned vest, the additional garment makes a tie bar redundant. You shouldn't wear your tie bar too high, or it won't serve its purpose.