You can install it horizontally, but there is no auto-rotate option for the video, meaning that your video will appear sideways in the app (i.e. when visitors are at your door). Since it is not really supported within Nest, this also means that features like motion and facial recognition won't work with your Nest Hello installed horizontally. This is unfortunate because if your existing doorbell is installed horizontally (for example because you have stone walls or uneven bricks), a Nest Hello won't really work as you'd want it to.