A collar pin, or collar bar, is a item of men's jewelry that connects the tips of the shirt collar and passes underneath the knot of the tie to elevate it above the front of the shirt. This accessory functions much like a tabbed collar in that it keeps the collars tidy and provides an aesthetically pleasing arc to the necktie.
There are three varieties of collar pins. My preference is the "barbell" style that I wore today (see the picture above). One end of the bar is unscrewed so that the pin will pass through special eyelets sewn into the collar. This style of collar pin almost certainly requires the purchase of bespoke shirts because it is nearly impossible to find a ready-to-wear shirt that features these eyelets. The second style of collar pin looks like a large safety pin. It may either be used on shirts with eyelets, or pierced through the collar tips of a shirt that lacks eyelets (the very idea makes me cringe). The third style is a bar with clips that grasp each collar tip; there is some danger that this style of bar may slip and go askew because the clips merely hold the collar ends with a friction fit.
Wearing a pinned collar is not for the stylistically tentative. Some people may find it elegant; others may think it's fussy. In either case, a pinned collar will likely draw attention because it is so rarely seen these days. To wear one you must be willing to suffer the consequences of being a non-conformist. A pinned collar is not fashionable. But it is stylish.