You learn an open G chord, feel proud of yourself for a week, and then a chart for a totally different song hands you a G chord that looks nothing like it — different fingers, different strings, sometimes a full barre shape further up the neck. Nobody told you it's still a G, so, reasonably, you assume you've been lied to. You haven't. Welcome to chord voicings, the part of guitar nobody explains before it happens to you.
A chord is a set of notes, not a shape
A G major chord is defined by three notes: G, B and D. That's it — that's the whole definition. Where those notes sit on the fretboard, which ones you double, and what order they appear in from low string to high string is called the voicing, and it's a separate decision from "which chord is this". As fretjam's guitar chord theory lessons put it, a voicing is "a particular expression of a given chord, based on the order in which the tones are stacked" — playing an open E major and playing an A-shaped barre chord at the 7th fret are two different voicings of the exact same chord (fretjam, Chord Theory Part 5: Chord Voicings). Same notes, same name, different outfit.
Why the same three notes get repeated so much
Guitar has six strings but most open chords only use three or four unique notes, so those notes get doubled — the same letter name shows up more than once, usually in a different octave. An open G chord, for example, plays G three times, B twice and D once across the six strings. That's normal, not a mistake: a single guitarist doubling notes is how a six-string chord fills out and sounds full instead of thin (see Gitori's glossary entry on chord voicing for more on doubling and note order). It doesn't matter which of the three notes you double, or how many times — as long as G, B and D are the only letters present, you're still looking at a G major chord.
The other reason two G shapes sound different: inversion
Sometimes what changes isn't the doubling but which note sits at the very bottom. In root position, the root note (G, for a G chord) is the lowest note you play. Move the third (B) to the bottom instead and you get the first inversion; move the fifth (D) to the bottom and it's the second inversion — the chord is still G, it's simply been reordered (Musictheory.education's lesson on triad inversions covers the mechanics in more depth, including how to hear the difference). A barre chord several frets up the neck can easily land on a different inversion of the same triad than the open-position shape you already know — which is exactly why it doesn't look, or sound, quite the same, even though it is.
The one takeaway to actually use
Next time a chart throws an unfamiliar shape at you labelled with a chord you already know, don't relearn the chord — check the letters. Work out (or look up in a chord diagram) which three or four notes the new shape actually contains. If they're the same letters as the chord you already play, you haven't met a new chord; you've met a new voicing of an old friend, and your ear already knows what it's supposed to sound like. For the deeper dive into doublings, barre-chord inversions and a full chart of triad shapes, Roberto's full guide to playing the same chord in different ways → is the next stop, alongside Triads and Inversions for the theory in full and Chord Names and Their Meanings if the letters themselves are still the confusing part.
If this saved you from relearning a chord you already knew, you can throw a coffee my way on Patreon — no pressure, purely for the fretboard diagrams.