> ## Documentation Index
> Fetch the complete documentation index at: https://docs.threesigma.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# 👋 Introduction

> Understand general concepts, response codes, and authentication methods.

## Base URL

Three Sigma API is built on **REST** principles. We enforce **HTTPS** in every request to improve data security, integrity, and privacy. The API does not support **HTTP**.

All requests contain the following base URL:

```
https://api.threesigma.ai
```

## Authentication

To authenticate you need to add an Authorization header with the contents of the header being `x-api-key sk_123456789` where `sk_123456789` is your [API Key](https://www.threesigma.ai/developer/account?tab=api-keys)

```
x-api-key: <x-api-key>
```

## Response Codes

Three Sigma uses standard HTTP codes to indicate the success or failure of your requests.

In general, `2xx` HTTP codes correspond to success, `4xx` codes are for user-related failures, and `5xx` codes are for infrastructure issues.

| Status | Description |
| - | - |
| `200` | Successful request. |
| `400` | Check that the parameters were correct. |
| `401` | The API key used was missing. |
| `403` | The API key used was invalid. |
| `404` | The resource was not found. |
| `429` | The rate limit was exceeded. |
| `5xx` | Indicates an error with Three Sigma servers. |

## Want to Jump In?

If you're ready to start using the API, check out the [Quick Start](https://www.threesigma.ai/onboarding) guide.

> [!TIP]
>
> If you're new to Three Sigma, we recommend starting with the [Playground](https://www.threesigma.ai/developer/playground) to get a feel for the data.

## FAQ

> **What is the difference between the API and the Playground?**
>
> The API is a programmatic way to access the data. The Playground is a visual way to access the data. The Playground is a great way to get started with the API.

> **How do you handle API versioning?**
>
> Currently, there’s no versioning system in place. We plan to add versioning via calendar-based headers in the future.


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