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Corrections first, questions second, everything else after that.
Corrections
If something here is wrong, tell us what and, if you can, under what conditions you measured it. Corrections are made in the article itself rather than appended as a footnote, because the article is meant to be the current state of the answer.
Questions
Questions about how something works are welcome and often become guides. Questions about which product to buy are not — we do not rank, recommend or price anything, and we will not make an exception privately that we would not make in public.
Supporting houses
The instrument-trade houses listed on the front page fund bench time and calibration. They have no say in what runs, they see nothing before publication, and there is no arrangement under which coverage can be bought.
Republication
Ask before republishing a guide in full. Quoting a paragraph with a link back needs no permission at all.