One of the most important terms to understand in Eurorack is HP, or Horizontal Pitch. HP is a unit of measurement that is used to describe the width of Eurorack modules. One HP is equal to 5.08mm, or 0.2 inches. Eurorack cases are sold in different widths, typical sizes are 60, 84, 104, 150 and 168 HP.
The height of eurorack modules are measured in "Rack Units" or 'U' for short. Both HP and U come from the standard for computer card racks. A standard eurorack module is 3U in height. This means that a 6U eurorack case has 2 rows of eurorack modules (6U case height / 3U per row = 2 rows of modules)
Most experienced eurorack musicians will recomend a 6Ux84HP or 6Ux104HP case to a beginner. When choosing a eurorack case, it is important to consider how many modules you want to include in your system. A small case will be filled quickly and limit functionality. A larger case can be overwhelming if you have never played the eurorack before. A 6Ux104 case will give you enough room for the essential modules, such as an oscillator, filter, envelope generator, and VCA while leaving enough room to add in effects and a sequencer.