This label on @derickr’s site is equally valid advice for both form fields :)

The text “will not be posted, please leave blank rather than typing garbage in though” is much closer to the “comment” field than the “email” field.

The gestalt principle of proximity rejects your internet comments! “Please leave blank, your comment will not be posted. Don’t type nonsense in though, that would be a waste of both of our time”

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