The frog helps the hairs of the bow achieve the necessary tension. It is not known how the frog received its name; in German it is also called “frog” (Frosch), whereas in Italian and French it is referred to as the “heel”. One possible explanation for the name is that the frogs on older styles of bows had a tendency to “jump” away, since they were not firmly attached to the stick; another explanation may be their shape, since from the side they can resemble a sitting frog.
Frogs have been used since the 16th century, and their invention was an important improvement on medieval bows, because now a bow could be tightened or loosened. The earliest form of the frog was the one which could be attached at any point between the stick and the hair. The invention of the frog is also relevant in the history of the instrument, since it facilitated the construction of flatter sticks.
In the late 17th century, luthiers experimented with frog designs that were intended to permit adjustable yet secure tension. One solution was the “crémaillère” bow, in which a metal loop or bracket made it possible to attach the frog at notches set at particular intervals from the end of the stick, thus creating greater or more gentle tension. Only a few years after this invention, the first frogs were created that featured a screw mechanism, laying the groundwork for the most common kind of bow used today.
Frogs are traditionally crafted from ebony, which is very strong yet easy to work and, last but not least, has a simply lovely aesthetic. As an alternative, even more valuable or more beautiful materials have been and are used, such as ivory, tortoiseshell or horn. The hank of hair is inserted into the hollow frog with a wedge, and then the slide, which is often ornamented with mother-of-pearl, closes the underside of the frog. The frog ring ensures that the hair is spread out properly and like the other metal parts mounted on the frog, it is often made of nickel silver or even genuine silver or gold (known, understandably, as silver or gold bows). The screw, which is known as the “button,” usually consists of the same metal and is commonly ornamented with ebony rings and a mother-of-pearl eye at the tip. As ornamentation, the frog has eyes on the side which are either simple round mother-of-pearl inlays or “Parisian eyes” with an additional ring. Frogs without eyes are referred to as “blank.” One especially interesting style is a “picture bow”, which features frogs that have eyes onto which tiny portraits have been mounted behind a miniature magnifying glass. These pictures usually show the master who made the bow, although they may also depict important figures in musical or instrument-making history.