[url=https://remote.rsccd.edu/en/musical-instrument-string-instrument-1283362/,DanaInfo=pixabay.com,SSL+]&quot;Guitar Fretboard&quot;[/url] by Pexels is in the [url=https://remote.rsccd.edu/publicdomain/zero/1.0/,DanaInfo=creativecommons.org+]Public Domain, CC0[/url]
Guitar strings that were carefully excited to contain only even harmonics.  Notice the node of the vibration in the picture.  You will witness such vibrations in lab today.