Calico is cross A with one more decision made. The two lower strings come up a whole tone exactly as in cross A, and then the top string comes down to C# — which is the third of A, and the note every other tuning on this site conspicuously lacks. The result is the only open set here that spells a real chord.
The retune, string by string
Derived from pitch arithmetic against standard GDAE. Three of the four strings move, and they do not all move the same way — two up, one down:
| String | Standard | Calico | Semitones |
|---|---|---|---|
| 4 (G) | G3 | A3 | up 2 |
| 3 (D) | D4 | E4 | up 2 |
| 2 (A) | A4 | A4 | stays |
| 1 (E) | E5 | C#5 | down 3 |
AEAC# — 3 strings move (4, 3, 1).
What the open strings spell
The distinct letters now sounding are A, E, C#, which reads as A major. Compare that with cross A, which is the same tuning minus this one change and reads as no triad at all. Adding the third is the entire difference between an open set that refuses to commit and one that has already decided. Tunes in calico sound settled where the same tunes in cross A sound suspended.
The drone, and the bow arm
The bottom two strings still ring as A and E, so the cross-A drone is intact underneath. What changes is the top: instead of doubling the E an octave up, the first string now adds the third above the melody. Brushing it while playing on the second string gives you a full triad from one bow stroke, which is why this tuning turns up in tunes that want a chord rather than a drone. The shuffle is what most players do with it.
What survives, and what does not
The standard double-stop library does NOT apply here — a stopped string moved, so any pair naming an interval against it now names a different one. Only string 2 held still, so this is the most disruptive tuning on the site as well as the sweetest-sounding. Treat it as its own instrument for the tune you need it for, and reverse it afterwards.
The chord it spells is the one on the A major page, and hearing that stated by four open strings is the quickest way to understand what the tuning is doing. Melodies come from the A major scale map; if the third turns out to be more commitment than a tune wants, cross A is one string away.