The decision about whether to play a half- or a whole-step at a certain place in the phrase can seem pre-determined because of scale relationships or the correlation between the improvised melody and the given harmony. Often the boundaries are self-imposed, however. Even the major scale contains two different areas in which half-steps occur (3-4 and 7-8). Also, the largest area of whole steps in a major scale occurs, not between 1-3 which might first come to mind, but between 4-7. Half-steps and whole-steps both deserve attention for their own sakes because of their inherent capabilities for tension and relaxation.
Of all intervals, the half-step contains both the greatest amount of tension and the greatest amount of relaxation, depending on its context. The simple melodic line shown in Ex. A below, uses only three pitches, D, E and F, which consist of a whole step between D and E and a half-step between E and F. Try for a moment to concentrate only on the two pitches of the half-step (try to disregard the pitch, D) and you'll hear the note E functioning as the tension pitch of the interval while F serves as its relaxed, or consonant, counterpart.
Ex. B uses the same half-step pitches but this time E becomes the consonant pitch and F becomes active, or the dissonant note of the interval. Listen to it here. (Still try to disregard the pitch, D.)
While the rhythms are the same in both examples, the changed tension/relaxation relationship within the half-step interval from Ex. A to Ex. B will occur even if the pitches are all played as equal durations. Sing Ex. A a couple of times with equal durations as notated below. Any idea why F sounds consonant while E seems dissonant? Now sing the equal durations of Ex. B below. Why do you suppose the situation is reversed?
A whole step, on the other hand, exhibits less energy than a half-step. Instead of portraying an intrinsic consonance or dissonance, it presents equality between its two pitches, and often relys on its relationships with half-steps to suggest a specific motion toward tension or relaxation.