Two notes are the chord
Play a C7 with both hands, every note you know. Now take away the root, the fifth, the ninth and the thirteenth. You are left with E and B♭.
Play those two alone. It still sounds like C7.
That is not a trick of the ear. The third says whether a chord is major or minor; the seventh says whether it is dominant or not. Between them they answer both questions a chord symbol asks. The root is being played by the bass. The plain fifth says nothing at all — remove it from almost any chord and the label does not change. The extensions are color, and color is a choice.
These two notes have a name: guide tones.
What they buy you
Three things, and the third is the one that matters.
Your left hand stops guessing. Two notes per chord is a decision small enough to make in tempo. Five notes is a decision you make at home and then cannot retrieve at a session.
Voice leading becomes visible. In a ii–V–I in C — Dm7, G7, Cmaj7 — the guide tones are F and C, then B and F, then E and B. Watch what moves: from Dm7 to G7 the C falls a semitone to B while the F stays put. From G7 to Cmaj7 the F falls a semitone to E while the B stays put. Two voices, one moving at a time, never further than a semitone. That is the entire ii–V–I, and your hand barely travels.
You hear them as a line. Once the pair is automatic, what you are following is no longer a sequence of grips but a two-part melody running under the tune. That line is what a soloist targets, and it is what makes a walking bass line sound like it knows where it is going.
The part nobody says out loud
Guide tones are usually taught as a starting point — something to use until you can manage "real" voicings. That is backwards. Listen to how much of Wynton Kelly or Red Garland behind a horn is two or three notes. The sparseness is not a lack of vocabulary; it is what leaves room for the soloist.
A rootless four-note voicing is not more advanced. It is a different job: it fills more space, and there are plenty of bars where filling space is exactly wrong.
Where to find them in the app
Switch the voicing style to Shell. What you get is the two notes above with the root underneath them — the root alone in the left hand, the third and the seventh together in the right. The root is there because a chord on a screen has no bass player; take it away and you have the pair from this article, exactly as it is written.
Then switch Voice-led on and step through the tune. The pair does not move house from chord to chord: one note stays where it is and the other falls a half step into its new job. That is the whole article, on the staff, at the speed you press the arrow key.
One detail worth knowing, because it is a real exception rather than a simplification: the plain fifth is dropped, but an altered one is not. In a half-diminished or a diminished chord the flat fifth is part of the answer to "which chord is this", so the app keeps it.
Try it on one tune
Take a tune whose changes you already know and play nothing but the shells through one chorus. One attack per chord, no rhythm, no extensions. It will feel thin for eight bars and then it will not.
Blue Bossa is the honest place to start: sixteen bars, one modulation, and two ii–V's whose guide tones move by step. When that sits, All The Things You Are is the test — it changes key four times, and the guide tones are the reason those moves feel like one road instead of four.