This antique / vintage ad was the front piece of a card advertising dog biscuits.
The ad served as a learning card for a musical term.
Since the card was about something of interest to potential customers
(educating their children in the finer points of music);
the back of the card advertising dog biscuits would have a good chance of being noticed.
The musical term for this card is "Decrescendo" - A decrease in loudness of a piece of music.
The memory aid is a dancing line that starts with a woman and decreases in height with girls that are smaller and smaller until the line ends with two little kitties with the cat at the end being the smallest.
Wellcome Collection
183 Euston Rd, London NW1 2BE, United Kingdom
©: listed as Public Domain CC mark 1.0
CC Mark 1.0 info can be found here
https://creativecommons.org/publicdomain/mark/1.0/