Thru is used on highway signage quite a bit, "Thru traffic must stay left" etc. No doubt it began as a necessity: fitting a long word on a constrained sign. But

Thru is used on highway signage quite a bit, "Thru traffic must stay left" etc. No doubt it began as a necessity: fitting a long word on a constrained sign. But its use by officialdom must have been hard for dictionaries to resist. I believe the -ough construct was one Noah Webster tried to purge from American English, along with ph- and the extra 'u' in au and ou. He succeeded on the last points but not on the former.