The Complete Idiot''s Guide to Music Theory

Part 5:
Embellishing
Now things get interesting. We need to come up with a second, higher, melody
that doesn’t clash with the lower melody—in fact, it should complement the first
melody. You can’t create this second melody in a vacuum; counterpoint is more
than just putting two unrelated melodies together. The two melodies have to fit
together logically, and have to work together to suggest the underlying chords.
So let’s start at the start. The lower part started on an F, which is the tonic of
the underlying key. According to our parameters, the upper melody can start
either on the tonic or the fifth. We don’t want to get too fancy with our first
counterpoint, so we’ll start the upper melody (the treble clef labeled “1”) on the
F two octaves higher.
In the first measure, the lower melody has a slight upward progression. To dis-
tinguish the upper melody from the lower part, we’ll give this second melody a
slight downward progression. Remember, we want the intervals between the
two voices to be pleasant-sounding, which means emphasizing the thirds and
sixths, and avoiding dissonant intervals. We also want to avoid parallel move-
ment, so we’ll leave the second note of the upper melody on F, which sounds
good against the D on the bottom. (It’s a nice minor third.)
For the third note, it’s time to introduce some motion—in this case, a down-
ward motion, to contrast with the upward motion in the lower melody. We’ll
move down from F to C, which is a nice singable fourth. It also forms a pleas-
ing minor sixth against the E in the lower melody.
The fourth and final note in the first measure also needs to show downward
movement, so we’ll continue down from C to A, an easily singable minor third.
The A also creates a pleasing major third against the F in the lower melody.
Now, if you look closely, you’ll see that not only did we introduce 
contrary
motion
in the upper melody (down, in contrast to the upward-moving lower
melody), but we also used the upper melody to suggest the underlying chord (F
Major). The downward melody is actually an arpeggiated F Major chord, in its
first inversion (F to C to A).
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The lower voice—the first of two parts in counterpoint.
Two melodies in
counterpoint that move in
opposite directions (one
up; the other down) are
said to have 
contrary
motion.