What is in vehicles, a swinging crossbar, to the ends of which other crossbars, or whiffletrees, are hung, to equalize the draught when two or three horses are used abreast called?

evener — In vehicles, a swinging crossbar, to the ends of which other crossbars, or whiffletrees, are hung, to equalize the draught when two or three horses are used abreast. (Webster's 1913.)