Rule of Thirds is a free Chrome extension developed by Ivor Potter. It falls under the category of Browsers and is a subcategory of Add-ons & Tools. This simple extension allows users to easily apply a 3 x 3 grid over images found on web pages, which are larger than 100px x 50px. The primary purpose of this grid is to assist photographers in visualizing the Rule of Thirds.

With Rule of Thirds, photographers can easily identify whether the main focal points of their photographs comply with the Rule of Thirds or not. By displaying the grid over the image, users can determine if their subjects are positioned at the intersection points or along the grid lines, enhancing the overall composition.

The latest version, v1.5, released on August 29, 2014, includes improvements such as ignoring images with "display: none" styles and ensuring that circles on intersection points are fully drawn.