Have you ever heard of customary business practices? They're sometimes called trade customs. They're basically the unwritten rules and ways of doing things that have become standard in a certain industry or trade. They’re not always written down laws, but they're widely known and followed in that industry.

Customary business practices can actually be a source of law in some situations. For example, if a contract doesn't say how disputes should be handled, a court might look at what's typically done in that industry to figure it out. Or, if a law doesn't give specific guidance on something, a court might look at how it's usually done in the industry.

Customary business practices can also be used to establish industry-specific regulations. Like, if a government agency doesn't have specific regulations for something, it might use what's commonly done in the industry as a guide.

Another example is International trade, where the International Chamber of Commerce publishes Incoterms, which are internationally recognised trade terms that serve as a basis for many trade agreements worldwide.

It's worth noting though, not all customary business practices are legally binding and a court or government authority might decide not to recognise a certain practice as a source of law. Plus, these practices can change over time as the industry, trade or technology changes.