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So, quite often I'll see something like: brandX.com, brandXinc.com, bluewidget.com, greenwidget.com all interlinked liberally creating huge back link profiles.
There's also that symbiosis of inter-linked sites under one umbrella where each of the separates are sufficiently unique, growingwidgets, sellingwidgets. cookingwidgets, etc. If it makes sense then that kind of link profile usually will NOT be penalized.
My theory (no testing) is that Google's penalty thresholds differ depend on niche.
It's a bit of a grey area as I don't want to add a nofollow link as that's supposed to be like a vote against a site from what I read.
Authority, trust, context. etc. might easily come into play, too.