Short for Horizontal Pitch. In the Eurorack format for synthesizer modules, the width of a module is defined as the number of hp (horizontal pitch) units. Each hp is 0.2” (5 mm). Most modules are even numbers of hp wide, although some are odd numbers. Also, modules tend to be ever so slightly less than exactly some multiple of 0.2” wide, just to make sure you don’t run into problems with ever so slightly too wide modules overlapping.
Those odd-hp-width modules were previously cursed, as it made it hard to make the edges of modules line up or leave no gaps in a row. (Some modular synthesizer enthusiasts are known to occasionally suffer from OCD.) These days there are more odd-hp-width modules to choose from, which helps restore order to the universe.
(One user wanted me to point out that HP also stands for Hewlett Packard, who he indicated was involved in the standardization of computer rack dimensions such as U which have been since borrowed by first pro audio and now synthesis. So there.)
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