Sacred Harp groups always sing a cappella, that is to say, without accompanying instruments. The singers arrange themselves in a hollow square, with rows of chairs or pews on each side assigned to each of the four parts: treble, alto, tenor, andbass. The treble and tenor sections are usually mixed, with men and women singing the notes an octave apart.
There is no single leader or conductor; rather, the participants take turns in leading. The leader for a particular round selects a song from the book, and "calls" it by its page number. Leading is done in an open-palm style, standing in the middle of the square facing the tenors (see: Leading Sacred Harp music).
The pitch at which the music is sung is relative; there is no instrument to give the singers a starting point. The leader, or else some particular singer assigned to the task, finds a good pitch with which to begin and intones it to the group (see: Pitching Sacred Harp music). The singers reply with the opening notes of their own parts, and then the song begins immediately.
The music is usually sung not literally as it is printed in the book, but with certain deviations established by custom; see: How Sacred Harp music is sung.
As the name implies, Sacred Harp music is sacred (Protestant Christian) music. Many of the songs in the book are hymns that use words, meters, and stanzaic forms familiar from elsewhere in Protestant hymnody. However, Sacred Harp songs are quite different from "mainstream" Protestant hymns in their musical style: they are often polyphonic in texture, and the harmony tends to deemphasize the interval of the third in favor of fourths and fifths. In their melodies, the songs often use the pentatonic scale or similar "gapped" (fewer than seven-note) scales.
In their musical form, Sacred Harp songs fall into three basic types. Many are ordinary hymn tunes, mostly composed in four-bar phrases and sung in multiple verses. Fuguing tunes contain a prominent passage about 1/3 of the way through in which each of the four choral parts enters in succession, in a way resembling a fugue. Anthems are longer songs, less regular in form, that are sung through just once rather than in multiple verses
It's so beautiful. Here are some videos of classic style Sacred Harp singers
Sacred Harp 159 Wondrous Love
Sacred Harp 146 Hallelujah, verses from Amazing Grace
Other links to really awesome Sacred Harp hymns:
What powerful music. I was thinking of maybe starting a Sacred Harp group in Nevada City... If you live here, would you be interested?