Do they go in alphabetical order?
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When the vote is called, the clerk goes through all the names in alphabetical order, but most do not answer her with their vote. Theoretically, if all the senators were standing around in the chamber, they would be voting alphabetically.
After she gets to the end, she just calls out each vote as it is cast.
This would also give the gutless wonders in the bunch a chance to see which way the wind is blowing, or sell their votes....