The original theory behind butted spokes was premised on the concept that a chain is as strong as it's weakest link. In a plain gauge spoke the elbow has about 80% of the strength of the rest of the spoke. Likewise the reduced diameter at the threads reduces strength. So 20% of the material throughout the length of the spoke is excess baggage.
In practice they deliver far more than the original expectations. A well selected butted spoke will outlast a plain gauge spoke of the same gauge (at the ends) significantly. It isn't often that less is actually more, butted spokes are one of the rare cases where it true.
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