What a copedent is
A pedal steel is played like a lap steel: a bar laid straight across the strings, the picks choosing which strings sound. What it adds is a row of pedals under the instrument and levers against the knees, and each one retunes a few strings while it is held. The chart of what they do is the copedent — the open tuning, then one row per pedal or lever, one cell per string it moves: a plus or a minus for every semitone, and the note the string reaches.
Two are charted here, each cell as a named source prints it: the E9 neck in the Emmons setup, and the C6 neck in the maker's factory setup. On each neck's page every change is set out string by string, every combination the knees allow is worked out into a tuning, and for each tuning the page shows what a straight bar holds.
How the changes combine
A copedent chart prints what each change does on its own. Players hold several at once, and four rules, each from a source, say what the strings do then.
- Each change moves only its own strings. Changes on different strings add up independently — the way Bobby Lee's "Understanding E9th" chord table plays every pedal and lever combination it lists.
- One lever per knee. A knee rests between its levers and moves one way at a time, and a lever with two stops sits at one of them (Wikipedia, "Copedent"). A foot has no such limit: two pedals can go down together.
- A shared pull. Two raises of one string to the same note put it at that note; they do not add. Two raises of one string to different notes are described by no source read here, so those combinations are named on the neck's page and refused — never guessed.
- The split. A raise and a lower of one string engaged together sound their sum, so the string lands between the two. Three examples printed on the Steel Guitar Forum were checked against it.
What a straight bar holds
With the changes resolved, each combination is simply a tuning, and a tuning is read the way every steel chord on this site is read: at a straight bar, every string whose note belongs to the chord sounds, every other string is left unpicked, and at least three strings have to speak (two for a bare fifth). Move the bar up a fret and every chord it holds moves up with it — which is why each neck's chord map is drawn at the nut, and why the finder can say, for any chord, at which fret and with which pedals the bar finds it.