Given the difficulties of engineering haptic displays, an alternative is to rely on real objects in the match zone to provide feedback to the somatosensory system. This is sometimes called a tangible user interface . As mentioned in Section 8.3.3, a powerful experience is made by aligning the real and virtual worlds. At one extreme, a see-through display, such as Microsoft Hololens which was shown in Section 1.2, enables users to see and interact with the physical world around them. The display simply adds virtual objects to the real world, or visually enhances real objects. Such systems are commonly included as part of augmented reality or mixed reality.