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The chord that points somewhere new
In C major, the only true dominant is G7, which pulls toward C. But songs constantly need pull toward other chords: toward Dm at the start of a phrase, toward Am in a bridge, toward F for a lift. A secondary dominant borrows the power of a real dominant and aims it at a chord that is not the tonic. In C major, A7 is the dominant of Dm, even though A7 is not in C major at all: A7 contains C#, a note outside the key. Play A7 followed by Dm and you will hear the same kind of resolution you hear from G7 to C, just aimed at a new target. That borrowed pull is the whole idea, and it is written V/ii, read as “five of two,” meaning the dominant of the ii chord.
Every diatonic chord except the tonic can have a secondary dominant, and this calculator lists all seven for your key. In C major they are: V/I is G7, the real dominant; V/ii is A7, pulling to Dm; V/iii is B7, pulling to Em; V/IV is C7, pulling to F; V/V is D7, pulling to G; V/vi is E7, pulling to Am; and V/vii is F#7, pulling to Bdim, which is rarely used because a diminished target is a weak landing place. Notice the pattern: each secondary dominant is a major or dominant seventh chord whose root sits a fifth above its target. That is the recipe. Take any target chord, go up a fifth, build a dominant seventh chord there, and you have its secondary dominant.
How to use this calculator
Choose your key and the tool shows the full table: each target chord, its secondary dominant triad and seventh chord, and the dominant’s notes. Start with V/ii and V/vi, the two most used secondary dominants in popular music. In C major, play Dm on its own, then play A7 followed by Dm, and feel how the A7 makes the Dm sound like an arrival instead of just another chord. Then try E7 followed by Am. That pair, V/vi to vi, is the sound of a bridge lifting into a new section. Once those two pairs feel natural, work through the rest of the table the same way: dominant first, target second, always listening for the resolution.
Secondary dominants also chain together. D7 pulls to G, and G7 pulls to C, so D7, G7, C is a chain of dominants falling by fifths into the tonic. Longer chains, such as E7, A7, D7, G7, C, walk the listener down a staircase of fifths, each step feeling inevitable. Jazz standards are built from chains like this, and recognising them turns a scary chord chart into a simple staircase. The progression analyzer will help you spot secondary dominants in songs you already play: any dominant seventh chord that is not the key’s own V is probably a secondary dominant, and its target is usually the very next chord.
Use them sparingly
The danger of secondary dominants is overuse. Each one adds tension and chromatic colour, and a progression full of them sounds restless, like a story that never stops introducing new characters. One secondary dominant per phrase is plenty in most styles, and two in a row should be a deliberate effect, not a habit. Also remember that the chromatic note in a secondary dominant, such as the C# in A7 within C major, needs care in the melody: if the melody sits on a C natural over that A7, the clash will sound like a mistake rather than a colour. Write or choose melodies that respect the dominant’s notes, or keep the dominant short, and the pull will do its work cleanly.
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