Cello inexpensive wood that has been blackened or "ebonized" to look like ebony, which is much harder and more expensive. Ebonised parts such as tuning pegs may crack or split, and the black surface of the fingerboard will eventually wear down to reveal the lighter wood underneath. Historically, cello strings had cores made out of catgut, which, despite its name is made from sheep or goat intestines which are dried out. Most modern strings used in the 2010s are wound with metallic materials like aluminum, titanium and chromium. Cellists may mix different types of strings on their instruments. The pitches of
Strings (tennis) or more of stringbed stiffness. However, many players who hit the ball hard enough to break the strings simply restring rackets whenever the strings break. The United States Racquet Stringers Association (USRSA) offers two levels of certification for stringers: the Certified Stringer and the Master Racquet Technician, of which around 350 exist worldwide. The Master Racquet Technician certification process includes testing of the stringer's ability to string a racquet, perform grip work, identify mistakes in an improperly strung racquet, and pass a written test that covers not only strings, but racquet technology as well. Since 2004, "RSi," the USRSA's monthly