The frizzen, historically called the steel, is an "L" shaped piece of steel hinged at the rear used in flintlock

Flintlock

Flintlock is the general term for any firearm based on the flintlock mechanism. The term may also apply to the mechanism itself. Introduced at the beginning of the 17th century, the flintlock rapidly replaced earlier firearm-ignition technologies, such as the doglock, matchlock and wheellock...

 firearms. It is positioned over the flash pan

Flash pan

The flash pan or priming pan is a small receptacle for priming powder, found next to the touch hole on muzzleloading guns. Flash pans are found on gonnes, matchlocks, wheellocks, snaplocks, snaphances, and flintlocks....

 so as to enclose a small priming charge of black powder next to the flash hole that is drilled through the barrel into where the main charge is loaded. When the trigger is pulled, the hammer or cock—which includes a shaped piece of flint

Flint

Flint is a hard, sedimentary cryptocrystalline form of the mineral quartz, categorized as a variety of chert. It occurs chiefly as nodules and masses in sedimentary rocks, such as chalks and limestones. Inside the nodule, flint is usually dark grey, black, green, white, or brown in colour, and...

 held into a set of jaws by a scrap of leather or thin piece of lead—snaps forward causing the flint to scrape down the face of the frizzen (historically called the 'battery'), throwing it back to expose the black powder in the pan. The flint scraping the steel causes a shower of sparks to be thrown into the pan, igniting the black powder and sending flames through the touch hole

Touch hole

A touch hole is a small hole, through which the propellant charge of a cannon or muzzleloading gun is ignited. In small arms, the flash from a charge of priming held in the flash pan is enough to ignite the charge within...

, which in turn ignites the main charge of black powder in the breech

Breech-loading weapon

A breech-loading weapon is a firearm in which the cartridge or shell is inserted or loaded into a chamber integral to the rear portion of a barrel....

 of the barrel, shooting the projectile out the barrel.

The development of the frizzen which combines both the "battery" or striking surface and separate pan cover on the less advanced "snaphaunce" lock is often credited to French gun maker Marin le Bourgeoys

Marin le Bourgeoys

Marin le Bourgeoys is most well known for inventing the flintlock mechanism that was used on weapons for over two hundred years....

 around 1610. He may have been influenced by the Spanish "miquelet

Miquelet

Miquelet Lock is a modern collector/auctioneer/curator term, largely used by and for the benefit of the English speaking world, widely applied to a distinctive form of snaplock, originally as a flint-against-steel ignition form, prevalent in the Iberian, Italian, and Balkan peninsulas, North...

" lock that utilized a similarly shaped frizzen at least two decades earlier.