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.FILL INDENT 6
	Continuing this approach, we find that the %2tritone%1 (augmented
4th or diminished 5th) may be thought of as the tonality-defining
interval, since its presence between the 4th and 7th is
unique in relation to all the other intervals found between the
degrees of a %2major%1 scale.
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Example 4
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	However, one more note must be involved so that we may be
made aware of the particular role of each part of the tritone.
Since, in a %2major key%1, one or the other of the parts of a tritone
must be the unalterable 4th of the scale, the whole step above it
%2must%1 be the equally unalterable 5th.
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Example 5
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	It follows that when a note is heard a half-step above either
part of a tritone, it must be the tonic.
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Example 6
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	Thus, in the major mode, we have two groups of three notes,
either of which may suffice to define a tonal center -- the 1st,
4th, and 7th, or the 5th, 4th, and 7th.  It must be noted that
these intervals need not occur between %2adjacent%1 tones only.
Other less critical notes may separate these scale degrees 
within a melodic unit.  An important thing to remember is that the
tritones formed as the result of the alterations (flattings) that
create the %2minor%1 scale do not serve the same purpose of tonic
definition.  Of course, composers often take advantage of this
obvious possibility of ambiguity in order to shift tonics.  The
"altered" notes in the original key become "unaltered" notes in
the new key, etc.
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Example 7
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	The next example shows how the various temporary tonics of
a somewhat chromatic line might be ascertained.  Among the notes
heard in measure 17 and the first two beats of measure 18, two
tritone relations may be found -- A%4F%1-D and F-B.