# Overview

The speckle viewer is developed on top of three.js (opens new window) and for the most part it behaves like a typical three.js application. However, Speckle viewer builds custom functionality on top of three.js when it comes to loading and storing data, object interpretation, scenegraph structure, and rendering.

The Viewer library is organized into viewer-core and extensions.

# Viewer-core

The viewer-core has it's own API and can be used as a standalone to develop applications. If all you want is a barebone web viewer, the viewer core is what you are looking for. Some of the features you'll be getting are:

# Extensions

In order to better customize the viewer functionality and behavior we've added support for extensions. A lot of explicit viewer functionality is provided via such extensions

Extensions are a way in which you can easily drop in additional functionality to your viewer instance. If the default behavior or functionality of an extension does not exactly fit your needs, you can easily extend on it and change the way it works.

Probably the most important aspect with extensions, is that you can develop your own! More details on this topic will follow in a dedicated chapter.