Subdirectories are named according to the family name of the fonts within.
Each family subdirectory contains the `.ttf` font files served by Google Fonts, plus a `METADATA.json` file with metadata for the family, and a `DESCRIPTION.en_us.html` with a description of the family in US English.
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The SIL Open Font License has an option for copyright holders to include a Reserved Font Name requirement, and this is option is used with some of the fonts.
If you modify those fonts, please take care of this important detail.
Source files for each family are often available from the designer, or from https://github.com/googlefonts
These fonts are usually the result of collaborative projects, where you are invited to discuss issues with the designers and even contribute to their on-going development.