Introduction to 3D Design - Autodesk AutoCAD 2013 Practical 3D Drafting and Design - page 4

Relative coordinates
Frequently, we want to specify a point in the coordinates, but one that is related to
the previous point. The format for the relative Cartesian coordinates is defined by the
symbol AT ( @ ), followed by increment values in the three directions, separated by
commas:
@X increment, Y increment, Z increment
Of course, one or more increments can be 0. The Z increment can be omitted.
For instance, if we define a point with relative coordinates, @0,20,10, this means in
relation to the previous point, 0 is in X, 20 is in Y, and 10 is in Z directions.