3c+4c begin on opposite side (1,2,3x,4x)
Note that both times the couple moving UP makes the arch.
In 17-32, the corners traditionally set one bar to the person you just passed, then one bar to the next person to pass. In a variant for [19-24] 1c turns BH in PdB.
Note: Cosh explicitly tells 1c to pass Lsh in [9-24]; however the Rsh pass is quite common, and nicknamed »Mairi's Divorce«.