The main distinguishing feature of the perfect cadence is the progression by falling fifth from the dominant chord - a major triad. This progression from dominant to tonic is generally considered to create a strong pattern of tension-resolution. The only similar progression by fifth from a major triad available from the diatonic scale is the progression from I - IV.
One of the simplest chromatic alterations is to sharpen the third of a minor triad from the diatonic scale so that it can function as a dominant. Examples b and c show how this alteration creates the effect of a V - I progression - it is almost as if the chord becomes a dominant onto a new and temporary tonic. This is why they are called secondary dominants - they behave in the same way as a dominant chord but are not the actual (or primary) dominant of the key.
One of the most common secondary dominants formed by sharpening the third of chord ii. It then acts, as in the example below as an intensification of the cadential progression ii-V-I.
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