Working your chest too much will pull your shoulders forward so you need to work the muscles that pull them backwards, i.e. the upper and middle back. Working on the lower back is obviously mandatory as well but that's more for the purpose of life itself.
If, on the other hand, you had been working your abs too much, you would need to work on the lower back.
Pull-ups are also very good for upper and middle back. Actually, any kind of pull works.
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I have just got home from work, before leaving work I was ready to go do some arms and chest. But then it started raining, like.. seriously! I got quite wet on my way home (bike) and now I can't find the willpower to go out in the rain again. Damn weather! At home I only have a 10 kg dumbell...