The seventh and eleventh string harmonics are substantially flat relative to equal temperament

Evidence

We know that some of the higher harmonics, starting with the seventh, can be quite a bit out of tune in any reasonable musical scale. The main deviants are the seventh harmonic, which is over 30 cents flat (see section 26.3), and the 11th harmonic, which is over 40 cents flat, compared to an equal-tempered scale. — Eric J. Heller, Why You Hear What You Hear- An…, 1870

Categorization

• QUESTION: Worth chasing: why exactly are 7 and 11 the most deviant harmonics, and how does this relate to tuning debates in other chapters?
• XREF: Connects to the physics of overtones/inharmonicity and equal temperament, core to the reader’s why-do-we-hear-what-we-hear question.