# Core Concepts

- [Getting started with an API project](https://docs.os.uk/os-apis/core-concepts/getting-started-with-an-api-project.md)
- [Authentication](https://docs.os.uk/os-apis/core-concepts/authentication.md)
- [Error codes](https://docs.os.uk/os-apis/core-concepts/error-codes.md)
- [Rate-limiting policy](https://docs.os.uk/os-apis/core-concepts/rate-limiting-policy.md)
- [OS API branding](https://docs.os.uk/os-apis/core-concepts/os-api-branding.md)
- [Styling OS NGD Data](https://docs.os.uk/os-apis/core-concepts/styling-os-ngd-data.md): Access resources for styling OS NGD data provided by the OS NGD APIs.
- [Creating a custom style for OS NGD API – Tiles](https://docs.os.uk/os-apis/core-concepts/styling-os-ngd-data/creating-a-custom-style-for-os-ngd-api-tiles.md)
- [FAQs](https://docs.os.uk/os-apis/core-concepts/faqs.md): Frequently asked questions on OS APIs.


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