The identity transformation is the default transformation between all coordinate spaces. The identity transformation, makes no change to the original coordinates of an object. This transformation is also referred to as the unity transformation, because of the mathematical matrix used to make this transformation.
The identity matrix looks like this:
┌ ┐ │ 1 0 0 │ │ 0 1 0 │ │ 0 0 1 │ └ ┘
(Transformations are accomplished with matrix multiplication; and 1, not 0, is the multiplication identity.)
If a transformation is not explicitly specified, the identity transformation is used to create that portion of the transformation matrix. Most transformations applied to a primitive can be in addition to, or instead of, the current transformation.