Double numerous notes of the chord at different registers. However, bear in mind that some notes of a chord will double well, whereas others, when doubled, seem to detract from the quality of the resultant sonority. A good example in terms of the major chord is the doubling of the third. When the third of a major triad is doubled, the third seems to stand out and imbalance the sonority. For this reason, within the study of functional harmony, a rule is often introduced that prohibits the doubling of the third of a major triad. The best notes to double tend to be the root and fifth because these give the chord its identity and backbone.
Another approach to the question of which pitch to double was suggested by Walter Piston. In his approach, Piston identified each degree of the scale as being either tonal or modal. The tonic, subdominant, and dominant scale degrees are tonal degrees while the mediant, submediant, and leading tone are modal. The supertonic is considered to function either way. Thus, instead of choosing the doubled pitch based on its position in the triad, one should determine the selection by the function of the pitch within the scale.
Quite obviously, sometimes you have—or want—to leave some notes of a chord unplayed. It’s not necessary to always voice every note of a chord. It’s okay to simply imply the full harmony by sketching in some subset of the total notes available. When writing for only two lines composers imply a triad by using only two pitches. When a major or minor triad needs to be implied, the better choice is to use the root and the third. When an augmented or diminished triad needs to be implied, the frequent choice is to use the root and the fifth together with the expected resolution. If a seventh chord needs to be implied the only, but ambiguous, choice is to write the root and the seventh. If the composer must represent a seventh chord using three pitches, the triad will be implied — as described above — and the seventh will be assigned to the third voice. For more complex chords, such as ninth chords, the fifth and seventh may be omitted but other substitutions are also found.