The Royal Navy's battlecruiser studies moved backward through the alphabet from G toward A, while the battleship sequence moved forward from N toward Z. Under that scheme, F followed G in the battlecruiser line and O followed N in the battleship line. An A3 would represent the far end of the battlecruiser sequence, while Z3 would do the same for battleships. The labels identify positions within parallel design studies rather than class names chosen for completed ships.
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How the G3 and N3 design letters worked
British battlecruiser studies ran from G back toward A, while battleship studies ran from N forward toward Z.