When building a device such as the Surface Pro and RT, Microsoft had many prototypes to help craft the final design. The prototypes, while not always pretty, give fantastic insight into the product design as it materialized deep inside the heart of Microsoft.
The best feature of the Surface are the Type and Touch covers that accompany the device. These products also went through considerable amount of R&D work to become the products that we can buy on store shelves today. You can see in the images below that the product had several working ideas for the touch/type covers including a hard, plastic-like, surface that is less flexible than the current offering.
These types of images offer a rare look into the minds behind the Surface and its development. While you can see the rough mockups are literally plastic and string, these are how products are developed to see if what looks good on paper, actually works in the real world. Naturally, by utilizing common products, these rough prototypes are less expensive compared to building out clay models and also take less time.
These are just a few of the images of the devices, hit up the source link for an even closer look at the development of the Surface.
Source: The Verge