When you create entities in a drawing, they are located in relation to the drawing's underlying Cartesian coordinate system. Every drawing has a fixed coordinate system called the World Coordinate System (WCS).
You can also define arbitrary coordinate systems located anywhere in threedimensional space. These are called user coordinate systems (UCS) and can be located anywhere in the WCS and oriented in any direction.
To specify points and distances using the keyboard you can use the following formats:

Cartesian coordinates: x,y,z

Cylindrical coordinates: R<alpha,z

Spherical coordinates: R<alpha<beta
Relative coordinates
If you place the @character in front of the entry, the coordinates are calculated with respect to the previous point. This technique is called: Relative Coordinates .
When Dynamic Dimensions are active and you type a value in the Length field and add a comma (,) the content of the Length field is copied to the command line and the @character is placed in front automatically, which allows you to specify the next point using relative coordinates with respect to the previous point.
Absolute coordinates
When a user defined coordinate system is active you can enter absolute coordinates (World coordinates) if you place an asterisk (*) in front. E.g. *0,0 refers to the origin of the WCS (World Coordinate System).
Working with Cartesian coordinates
In the Cartesian coordinate system we use three perpendicular axes: the xaxis, the yaxis and the zaxis. All axes originate in the origin point of the coordinate system. The xaxis and the yaxis define a horizontal plane, while the xaxis and the zaxis and the yaxis and the zaxis define vertical planes. A point is defined by its distances to the yz, xz and xy planes. These distances are called the xyzcoordinates of a point.
If you want to enter the absolute Cartesian coordinates of a point, type the x, y and z coordinates separated by commas: 45.5,57.3,60
If you omit the zcoordinate, the point is placed in the xyplane (Z = 0).
If you place the @character in front of the entry (@x,y), the coordinates are calculated with respect to the previous point. This technique is called Relative Cartesian coordinates .
Using relative Cartesian coordinates to draw a rectangle
 Launch the RECTANGLE command.
 Specify the first corner of the rectangle.
 In the command bar type: @<width>,<height>
 <width> = the width of the rectangle in drawing units, measured along the xaxis
 <height> = the height of the rectangle in drawing units, measured along the yaxis
Working with cylindrical coordinates
In a cylindrical coordinate system we use three perpendicular axes: the xaxis, the yaxis and the zaxis. All axes originate in the origin point of the coordinate system. The xaxis and the yaxis define a horizontal plane, while the xaxis and the zaxis and the yaxis and the zaxis define vertical planes.
A point is defined using the following format: R<alpha, z.

R = distance to the origin in the xyplane

<alpha = the angle between R and the xaxis (positive angles are measured counter clockwise)

z = the height above the xyplane.
If the zcoordinate is omitted, cylindrical coordinates are referred to as polar coordinates.
If you place the @character in front of the entry, the coordinates are calculated with respect to the previous point. This technique is called Relative Cylindrical coordinates .
Working with spherical coordinates
In a cylindrical coordinate system we use three perpendicular axes: the xaxis, the yaxis and the zaxis. All axes originate in the origin point of the coordinate system. The xaxis and the yaxis define a horizontal plane, while the xaxis and the zaxis and the yaxis and the zaxis define vertical planes.
A point is defined using the following format: R<alpha<beta

R = distance from the origin

<alpha = angle in the xyplane (positive angles are measured counter clockwise)

<beta = angle measured from the xyplane (positive angles are measured counter clockwise, above the xyplane)