Double bass virtuoso Božo Paradžik wrote a very cool post about the history and differences between hairpins and crescendos and diminuendos. Here’s what he wrote, and be sure to check out the original post here on Instagram.

Božo Paradžik on Hairpins
Hairpins are nowadays generally considered a synonym for written crescendo or diminuendo. But it is rather a result of rapidly changed performing practices in the early 20th century than a continuation of the traditions. Throughout the 19th century hairpins were used by many composers for affecting tempo and agogics as well.
Johannes Brahms became a legend still during his lifetime. All he said, or did, or performed was carefully documented. I will note now a few details that be useful as a reference for many other pieces composed at the time:
Brahms used in his chamber music the hairpins mainly for slight tempo changes. Of course: intensity of the expressions, articulations and the dynamics were affected by the hairpins as well, however the intensity dependended a lot of the specific content and of the main dynamic of that fragment. When he was performing his own compositions, at the end of the decreasing hairpins, Brahms would usually take more time to finish those musical sentences “like if it was a little fermata”.
Check all romantic pieces you know and try to imagine them again under these circumstances. Not all, but many composers used hairpins different way than it became common in the 20th century. Double bassists for example could take a new look at Koussevitsky’s bass concerto with this new perspective, for he was also a typical example of a composer who used hairpins primarily for tempo marks, just like Brahms did. It will become a different piece and could sound much more natural and logical that way. 😊
Many other composers in middle or late in 19th century used the hairpins similar way, tempo was generally considered as something more flexible and shapeable if compared to our time, where tempo became something quite strictly metronomical…. and hairpins became just an indication of the loudness.
Božo Paradžik