This page is a tool I use for finding chords that are "close" to another chord, but also have the desired character that I want.
Instructions: Click on the piano-like display to select or deselect notes for the chord that you are interested in. Click on the table headers to sort. That's it.
The piano-like display represents the frequency of the note on a logarithm scale. The tuning is set to Just Intonation currently (as opposed to Equal Temperment), hence the slightly uneven widths of the keys. A future enhancement will be to allow the selection of different tuning systems to see how that effects the results.
The small circles under the keys indicate the selected notes.
The vertical lines underneath the keys indicate the harmonics (or overtones) of the sound produced by the notes of the selected chord. This lets you see how the harmonics of the notes support or clash with each other. A future enhancement will be to allow the selection of a variety of timbres to see how that effects the qualities of chords.
A text summary of the selected chord is next. The notes are listed, followed by the parameters Dissonance, Tension, Instability, and Modality. Briefly, dissonance is the opposite of "niceness", tension is a measure of how a chord is drawn to a specific key, instability is a combination of these two numbers that closely matches empirically measured scales of perceived niceness, and modality represent how "Major" or "Minor" a chord sounds.
I read a very interesting article called "Harmony Perception: Harmoniousness is more than the sum of interval consonance" by Norman D. Cook in the journal Harmony Perception from February 2009. The coolest thing about this article is that it explained much of the experimental evidence related to 2 and 3 note chords without requiring "the entire edifice of Western music theory". The numbers on this page are based upon the work in that article. I have modified the calculations slightly to attempt to normalise the values so chords of different numbers of notes can be compared.
The table at the bottom contains all the "nearby" chords to the selected chord, with their parameters. I define a nearby chord as a chord with the same number of notes as the selected chord, with at least one note in common, and all the other notes must be at most a tone away from the starting note. This can produce a large number of possible notes in the table, so it will keep growing until it is finished. The table is sorted by "Instability". The selected chord is shown as a green row.
You can sort the table by clicking on the header of the column that you want to sort by.