Change of clothes, lunch, standard flat repair stuff, rain gear. Sometimes my laptop too if I've needed to take it home for the night/weekend. For 12 miles each way, even just the weight of clothes, rain gear and lunch on my back gets annoying, hence the panniers. Also gives me the space to be able to stop and grab a few things at the shops on the way home.
Shoes, belt & deodorant stay at the office.
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