Brand Guidelines
Brand guidelines are the rulebook governing how a company's visual identity is applied: logo variants and clear space, colour values for screen and print, typefaces and sizes, imagery, and worked examples. They make design decisions checkable and independent of any single person.
In practice
Brand guidelines only help if they are used; an 80-page PDF in a shared folder changes nothing. A short, digitally accessible version covering the ten rules that actually come up day to day works better, along with ready-made templates for quotes, presentations and social posts. The most common mistake is listing only prohibitions; examples of what correct looks like are far more useful. Nominate one person who decides in borderline cases, otherwise the loudest opinion wins. For websites, a design system extends the guidelines with components developers can use directly.