Strobing FIT Nock: Designed to fit nearly every hunting arrow on the market the Nockturnal Fit strobing base nock will fit X-diameter arrows, and each three-pack of Fit nocks includes three sets of Nock Collar adapter sleeves to fit GT, S and H arrows. Each Fit nock features ultra-strong, impact resistant, clear polycarbonate nock construction encapsulating two brilliant LED lights that alternate after engagement by the patented string-activated, piston-driven linear contact switch. Like all Nockturnal nocks, the strobing Fit nock is guaranteed to reliably activate immediately when the string is released, and it can be switched off quickly and easily with the Nockturnal Nock Tool, a small screwdriver or most broadhead tips via an opening in the side of the nock. With well in excess of 20 hours of lithium battery life, each strobing Nockturnal Fit weights approximately 25 grains and will remain brightly lit and strobing for plenty of time for arrow retrieval and can be easily spotted in daylight or darkness. The strobing version alternates 400 times a minute between Red/Green.