Start with a plain progression in C major, then reharmonise it. Hear the before and after, see exactly what changed and why.
Tap a Roman numeral, then tap a slot. Or load a preset.
Secondary 7ths slip a dominant 7th in front of each chord, borrowing the pull of a perfect cadence to tonicise it. Tritone subs replace that dominant with the one a tritone away — same 3rd and 7th, but a chromatic bass. Relative swaps trade a chord for one a third away with the same function (I↔vi, IV↔ii, V↔iii). Borrowed chords reach into C minor for a darker colour (iv, ♭VI, iiø).