Your main problem, if you look on the internet for example, is that there is too much choice and it's hard to know which one would be exactly right.
Don't know if you actually have a problem with your back, if you have, then a pillow will only be treating the symptoms. If you want to treat the underlying cause I'd recommend seeing a chiropractor... he/she may also give some advice on a pillow.
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Steve J
Unkind tongue, right ill hast thou me rendered
For such desert to do me wreak and shame