Look at the Mix Pyramid. Notice that the top or focal point of the pyramid is the lead vocal. The lead vocal or soloist must always be on top and be able to be understood by someone who does not already know the words to the song. Just under the lead vocals come the backing vocals or choir. They are the first layer of support, and they too need to be audible as a distinct musical element that helps convey the message of the song.
Beneath the vocals come the instruments that provide fill and color. These add musical interest and highlights, and in fact may be the most prominent portions of the mix when the vocalists are not singing. Instruments might include Lead Guitar, Synthesizer, Brass, and/or Percussion. These instruments will often lead between vocal lines or during instrumental breaks and may need a bit of help from the Mix Musician to make sure their parts are heard when needed.
The primary bed of chords that form the harmonies under the lead parts of the music are often laid down by instruments such as rhythm guitar, piano and/or electronic keyboards. These instruments should be audible but never dominant in the mix.
Lastly low frequency instruments add foundation and weight to the music. These include drums, bass guitar, organ pedals, and the low end of electronic keyboard instruments. While almost never dominant in the mix, if they are not given their proper place and balance in the mix, the music will sound thin and will lack much of its grandeur.