I think you misunderstood what I was saying, Jack. If you capo up and play the same chord shapes, you have the same open strings, just a bit higher, and you are of course playing in a different key.
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"if you always start on a particular note, you're always singing in the same key. " But of course that note doesn't tell you what the key is, and knowing the key doesn't tell you what the start note is. It's the end note that you use to determine the key (most of the time). The start note can be just about anything, depending on the tune.