Dominant without the root
Indeed, it's always good to know that the dominant of any given key can be replaced with the VII degree chord based on the leading-tone. You might recall that the latter includes the tritone (refer to article 1 and article 9) and that apart from the root, the 7th degree triad has all the notes of the dominant seventh chord, as you can see in the following example:
That's why the triad of the 7th degree is often considered a dominant without the root. Incidentally, substituting the V degree with the VII degree constitutes an exception to the rule that we are about to see.
Relative chord substitution
When studying the topic in article 3, you learned that two scales that are relative to each other share the exact same notes, but their tonics are separated one and a half tones. From this, you could conclude that any chord from any given key can be replaced with its relative major or minor (see, for example, the substitution of the VI degree with the II degree in the IV-V-I progression in article 14)
The table below summarizes the possible substitutions in this regard:
As I mentioned earlier, the V degree is an exception to this rule, so it's not on the table. Its relative chord would be based on the III degree. Yet, despite its status as the relative to the V degree, the III degree is in fact considered a tonic chord (see article 14). So it can't be used to replace a dominant chord. It makes sense, doesn't it?
And even more so when you consider seventh chords. In this case, the dominant chord includes the famous tritone (see article 9), which does not correspond to the seventh chord based on the 3rd degree.
But you can take chord substitutions even further. In article 15 we saw that in the II-V-I progression, you could replace the II degree with a secondary dominant based on the same root. Well, guess what? You can replace any degree chord of any scale with its equivalent on the parallel scales. FYI, the "parallel scales" are those which share the same tonic. Thus, the E major, E natural minor, E harmonic minor, and E melodic minor scales are all parallel scales of E.
Generally speaking, though, you should not replace a major I degree chord with its harmonic equivalent at the end of a perfect cadence, unless you want to produce a radical change to the character of the song.
Below are two examples of tonal substitution:
Nevertheless, I obviously encourage you to experiment with other progressions and substitution options! As you have surely realized by now, adding non-chord tones to the key results in new colors and characters, which might make you want to occasionally modulate towards another key.
What's that, you don't know what "modulating" means? Don't worry, we'll talk about it in the next article...!