The early Shearwater Research dive computers were designed by Bruce Partridge, the founder, with a 1.8 mm tool slot on the battery cap for installation and removal that at the time was the thickness of a Toonie, a Canadian two dollar coin. The modern Shearwater dive computers that incorporate a removable battery, such as the Perdix and Petrel, still have a slot that same thickness. If you use thinner coins such as a US quarter dollar, or other makeshift solution, you may eventually strip the edges of the slot. Shearwater has for years included this battery cap tool with every computer model that requires it, but if you lose yours or just want an unusual key fob, this is the OEM factory tool in anodized aluminum with the Shearwater logo.