While you might find slide rules posted at a water park, back in the 50s and 60s, slide rulers were the precursor of the pocket calculator. With a few slips of these wooden calculators, you were able to solve complex mathematical equations. And, while you couldn’t get the exact answer, you could get a close estimate.
Now, since the accuracy of these devices was dependent on their size, most of them were well over a foot long. So, in order to make them easy to transport, many of them came with leather holsters that clipped to your belt. So, if you were walking down the street and were inspired with a mathematical problem, you could whip out your slide ruler and have at it.
If you were a nerd, you had a slide ruler. And if you were a super nerd, your slide ruler hung off of your belt. But, if you were a premo super nerd, your slide ruler hung down almost to your knees.
The HP 35 pocket calculator was not only a major scientific breakthrough, it destroyed a whole subculture.