Usage Rule: Something is illegible because it is impossible to read; it is handwritten or printed poorly. The letters cannot be understood. → The letter quickly became illegible in the rain.
Something is unreadable because it is uninteresting, dull, or badly worded.
→ The novel was unreadable. I fell asleep twice trying.
Still, unreadable is sometimes also used for illegible, as a synonym, and for other things one cannot understand, such as a facial expression or emotions.
→ She had an unreadable expression on her face.
Advice: It’s best to keep the distinction and not use unreadable if something is illegible. I hope this explanation is readable.
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