“A style guide or style manual is a set of standards for the writing and design of documents, either for general use or for a specific publication, organization or field.

The implementation of a style guide provides uniformity in style and formatting within a document and across multiple documents.”

Source Wikipedia

Here are some examples of style guides being used out there in the wild.

• Clearleft Styleguide
• Mailchimp Patterns
• Code for America
• A Pattern Apart
• Salesforce
• Style Tiles
• Style Guide Boilerplate
• Barnard Levit Style Guide
• Starbucks - Mockups
• Starbucks - Style Guide
• Design Systems: Building for the Future

Keep it simple and consistent.

Update: 2014-06-01

• A really good example from Font Shop

Update: 2015-04-23

• CodePen style guide