It is specified in R4RS and IEEE standard P1178. Scheme is often used in computer science curricula and programming language research, due to its ability to represent many programming abstractions with its simple primitives. A Logo user or program manipulates the graphical turtle by telling it to move forward or back some number of steps, or by telling it to turn left or right some number of degrees.
Turtle geometry has two major advantages.
One is that many paths are more simply described in relative than in absolute terms. For example, it's easy to indicate the absolute coordinates of the corners of a square with vertical and horizontal sides, but it's not so easy to find the corners of an inclined square. We chose Scheme as the programming language because of its educational value, as noted before. We added Turtle Graphics because of its ability to visualize computations, and thus help understanding them better. Second, we wanted to add parallel programming.