The blog Core77 shows us this curiously designed baseball cap. It has notches cut into the brim, one on either side. Why? The answer is below the fold.
It's to accommodate wrap-around sunglasses. Paul Cunningham, the founder of the company Notch, served in the US Marine Corps. Like many servicemen, he wore wrap-around eyewear and found it irritating when baseball caps pressed against the frames of his glasses.
So Cunningham founded his company to fill this niche market, which includes competitive shooters and other athletes who like to keep the sun out of their eyes, no matter which way they are facing.
Notch also offers baseball caps with openings for ponytails, too. Presumably this will also work for your manbun.