Ensembles (Groups)
String Quartet
It is common for composers to use combinations of instruments from the same family. Each family has instruments of varying size and therefore varying pitch. The larger the instrument, the lower the pitch.
A String Quartet consists of 2 Violins, a Viola and a Cello. Note there is no Double Bass.
Wind Quintet
A wind Quintet consists of the 4 instruments of the Woodwind section (family) and the French Horn, which is a Brass instrument but sounds equally well when combined with the Woodwind instruments.