As it was explained earlier.....They are not allowed to miss without being excused. However, if an important/close vote is being held, they may find a member (usually of the opposite party) who will cancel out their vote and both not show/vote as a courtesy and both be excused. The posted gave the name of the practice. Learn something every day on FR.
Since the Senate seems to have so few courtesies left, I guess we should be happy that this one is still intact.
Thank you for the explanation. I wonder what Cboldt thinks?
they called it vote pairing