Principles of Part-writing
Traditionally, in the study of music theory, the principles of part-writing have been presented in  four-part chorale style.  In this structure, the parts are referred to in the familiar terms of Soprano, Alto, Tenor, and Bass (SATB).  The principles learned, however, are easily translated to virtually every medium of composition.

Because so much attention has been paid to four-part textures, a widely accepted terminology has been developed.  Two typical structures are:
     Close structure:  an octave of less between the Soprano and Tenor
     Open structure:  more than an octave between the Soprano and Tenor

When writing in chorale style, various guidelines are necessary for smooth and strong voice-leading within the parts.

1.  Nothing should be written above the Soprano or below the Bass
2.  The Alto and Tenor parts can cross, but only briefly, and only for reasons of strong voice-leading
3.  Avoid more than an octave between adjacent upper voices
> No more than an octave between the Soprano and Alto
> No more than an octave between the Alto and Tenor
> More than an octave between the Tenor and Bass is acceptable

4.  Avoid parallel 5ths and 8ves

5.  Avoid direct 5ths and 8ves in the outer voices

Often it is necessary to double or omit certain chord members to maintain strong voice-leading.  When doing so, a few guidlines should be considered.
1.  Try to double the root or third  (Double the fifth only if strong voice-leading prohibits doubling the
     root or third)
2.  Do not double active (tendency) tones
3.  Omit the 5th if necessary
> Omitting the root changes the chord entirely
> Omitting the third removes the quality of the chord (major, minor, etc.)