One other variation on the theme worth mentioning is the Audemars Piguet Royal Oak Double Balance Wheel. Here there are two balances, however,
link they're on the same balance staff and cannot oscillate independently. Each balance has its own balance spring, with inner and outer pinning points opposed to each other by 180º – the idea is that the two springs will oscillate perfectly symmetrically and once again, tend to cancel out
link rate
link variations.
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