Counterpoint is the relationship between two or more melody lines that are played simultaneously, and the study of counterpoint is a tried and true method of developing a deeper understanding of harmony and voice leading.
Species Counterpoint requires adherents to abide by a set of strict rules that specify how the melody lines can move and interact.
Let's start with a few pertinent definitions:
Consonant intervals: perfect unisons, octaves, fourths, fifths, and major/minor thirds and sixths.
Dissonant intervals: tritones, major/minor seconds and sevenths.
Parallel motion: both voices move in the same direction by the same interval.
Similar motion: both voices move in the same direction but not by the same interval.
Oblique motion: one voice moves and the other voice remains on the same tone.
Contrary motion: both voices move in opposite directions.
Intervals are labeled with a number 1-8, and it is not necessary to specific whether they are major or minor. Intervals larger than an octave are often abbreviated, e.g. a 10th would be labeled as a 3rd; a 12th as a 5th, etc.
Similar or parallel motion to a unison or octave should be avoided, except to the last interval.
The following piece is an example of first species counterpoint that follows is composed entirely with consonant intervals. Form: abac
And this next piece is another example of 1:1 counterpoint that utilizes identical rhythms, consonant intervals, and bitonality, i.e. the top voice is uses the tones from the E Phrygian scale, and the bottom voice uses the tones of the C "Lixian" scale (lydian + mixolydian).