There are several distinct systems for classifying musical instruments into groups. One of the most common assigns instruments to one of four groups, called "families." All instruments in a common family make sound in the same way, although they may make dramatically different sounds. Some instruments are difficult to classify in this system, either because they have characteristics of multiple families or because they don't easily fit into any of them.
Brass instruments are played by blowing into a mouthpiece while making a "buzzing" motion with pursed lips. To play different notes on a brass instrument, players can modify their air flow, the tightness of their pursed lips and the positions of one or more valves on the instrument. Common brass instruments include trumpets, trombones, tubas and French horns. While many brass instruments are actually made out of brass, instruments made of numerous other materials are considered brass instruments because they produce sound via the same process.
Woodwind instruments are also played by blowing into a mouthpiece, although musicians do not buzz their lips in this case. The musician's air flow passes over a wooden reed or across a fine edge, such as with flutes, recorders and whistles. Reed-based woodwinds include clarinets, saxophones and bassoons. As with brass instruments, the term "woodwind" does not mandate the types of materials used to create the instrument itself.
Percussion instruments are played by striking a resonant material with a hand, drumstick, mallet or other specialized instrument. All drums are percussion instruments, as are cymbals, xylophones and other similar mallet instruments, tambourines, maracas and even wind chimes. When drums are struck, their vibrating heads produce noise, sometimes enhanced by snares that are pulled tightly across a parallel resonant surface. Wood blocks and wooden marimba keys also vibrate to produce distinct tones, as do metal percussion instruments like triangles and xylophone keys. Some categorize pianos as percussion instruments because of their internal mechanisms of striking hammers and the fact that they are played by "striking" the keys with the fingers.
String instruments have a series of taut strings that are plucked, strummed or stroked with a bow to produce sound. Examples of string instruments include the violin, viola, cello, bass, guitar, bass guitar and harp. Even electric guitars and bass guitars are considered members of the string family. Most string instruments have "necks" along which the strings are pulled, and musicians achieve a wide range of tones by pressing strings down onto the neck in specific positions while playing them. Because acoustic pianos ultimately produce their tones through a series or vibrating cords, they are often considered members of the string family when not otherwise categorized as percussion instruments.
While the piano can arguably fit into more than one category, many modern instruments do not fit into any of these categories. Synthesizers, while having the appearance of pianos, do not produce sound via any of the processes covered by the four families of instruments. It's not just the modern instruments that are difficult to classify; the accordion is sometimes categorized as a woodwind because it uses an internal reed and moves air through the use of bellows. Other times, it is grouped with pianos, organs and other keyboard-based instruments in the percussion family.