Early Second Life avatars, from 2003 onward, were constructed using several basic items that defined their appearance and functionality. This system is still available today, though more modern technologies exist and mostly superseded it. But even the most modern alternatives are often still wholly, or in parts, backwards compatible to this early system. As the availability of modern alternatives varies widely between commercial Second Life and the more experimental and research focused, noncommercial fork of OpenSimulator, there is a huge motivation for the Shork Group to favour the tried and tested and reliable basic system.

Now, let's take look at the two critical components that are needed to form a basic avatar inside Second Life and OpenSimulator alike.
The two critical components are the Shape and the Skin.
Here’s a description of each and their functions:

The Shape item defined the overall structure and proportions of an avatar. It controlled a wide range of physical characteristics through adjustable sliders, including:

• Height and Build: Adjustments for overall stature, body thickness, and leg length.
• Facial Features: Customizable attributes like eye size, nose shape, lip fullness, and jaw structure.
• Body Proportions: Options to tweak torso length, hip width, arm length, etc.
• Gender and Anatomy: Basic biological and gender-defining characteristics (e.g., breasts, hip curves).

The Shape item was essentially a blueprint for the “skeleton” and 3D mesh that defined how the avatar's body would look in-world. Users could edit this item to achieve a personalized appearance, and once saved, the shape could be worn or shared.
Without a shape, the avatar would default to a generic appearance or just form a a diffuse cloud.

The Skin item was a texture layer applied to the avatar’s shape, giving it color, detail, and individuality.
Key aspects of the Skin included:

• Surface “Texture” (as in structure): The skin determined the visual appearance of the avatar's body, including skin tone, ruddiness, and other surface details.
• Facial Features: It included painted-on elements such as eyebrows, lip color, and eye shading.
• Detailing and Realism: Skin textures provided shading, highlights, and contours that simulated realistic human (or non-human) appearances.

The Skin was essentially the “paint” over the “canvas” of the Shape. Together, they determined whether the avatar looked like a cartoonish character, a realistic human, or even a fantastical creature. Years later, from around 2006 onward, a so-called “tattoo-layer” was added to the skin. Three images with a maximum resolution of 2048*2048 pixel could be added, one for the face, one for the torso and arms, and one for the beltline / waist and downwards.
These allow, up to the present day, to create finely textured Skins.
Without a skin, an avatar would default to either a gray figure, an orange cloud or be invisible, depending on the software used to access Second Life or OpenSimulator.

While the Shape dictates the form and dimensions of a standard avatar, the Skin defines the outer details, effectively “wrapping” the shape with a detailed texture.

For example:

• A tall, muscular Shape could be paired with a green Skin for a reptilian-like appearance.
• A petite, slender Shape combined with a freckled Skin might create a youthful human character.
• By using a buff shape and adding suitable textures to the three regions of the tattoo-layer of a green Skin, a scarred orc with tattoos can be created.

Together, they form the foundation of an avatar's look in Second Life, as well as OpenSimulator, before clothing, hair, and other accessories are added. All components designed to match the default avatars design principles, i.e. are designed as textures to be applied to system default components, have the advantage of being automatically rigged and conforming perfectly to the system default avatars motions.

• Whilst there exist “avatar bodies” as .blend and .DAE files, they are primarily references for the size of objects and can be used as a “frame” to apply the reference UVMaps to as to be able to paint the three “tattoo layer” textures for the Skin item, that can be applied to the Skin item to form an avatars Skin, in a comprehensible way.
• Separate textures for the tattoo-layer are needed as soon as the skin should be anything other than unicolor.
• Textures deviating from the reference UV maps for the Skin items tattoo layer in layout will not be properly displayed on the resulting avatar, once a Shape item is applied to create a visible avatar.
• Creating a body, or using anything other than the reference UV Maps, requires to not only rig the newly created body the the skeletal model, it also requires either creating anew or re-rigging the many thousands of already existing components such as
  • eyes,
  • hair,
  • pants,
  • socks,
  • shoes,
  • trousers,
  • skirts,
  • glasses,
  • hats,
  • shirts, …
• … as the avatar would not be compatible with any of the existing components.