The Loudest Warning format is a set of standards for panel size and mounting holes used for creating individual modules at the 4U (7" tall) size

The most popular 4U synth is the Serge synthesizer, which was based on a full width (17" x 7") panel. Multiple modules are laid out on the panel and built behind the full width panel. It's a great system... but once built, you can't move the modules around. And you need to select a full panels worth of modules before hand. No adding just another filter or one oscillator later.

4U modular synths should be... modular!

The LW format was discussed and created in THIS thread on the Muff Wiggler Synth DIY forum. Charlie Kerr came up with a set of standards that at a minimum allow for uniform mounting and spacing, all other details of module design is left up to the designer. Carlie discusses the standards, and has info on his LW panels HERE and HERE.

The 4U boat available here conforms to the LW standard. The enclosure is 17" wide and 7" tall. Mounting holes are available at every inch on center, and are 169mm apart top to bottom. The rest of the design decisions are left up to the designer or DIY builder.