Absolute & Relative Co-ordinates

Both Cartesian and polar co-ordinates come in two flavours, absolute and relative. The distinction is quite simple, absolute co-ordinates relate to the X and Y axes and the origin of the current co-ordinate system, whilst relative co-ordinates relate to the current pick point. When you are specifying co-ordinates you need to tell AutoCAD which type you want. Absolute co-ordinates are typed exactly as in the examples above. To specify a relative co-ordinate you need to use the "at" symbol as a prefix. In the case of the two examples above a relative Cartesian co-ordinate looks like this @34.897,45.473 and a relative polar co-ordinate looks like this @34.897<30. Relative co-ordinates are very useful for drawing objects which you know the size of. For example, you could draw a square of 12 units with its lower left hand point at 30,40 as follows:

Command Sequence

Command: LINE
From point: 30,40 (an absolute Cartesian co-ordinate)
To point: @0,12 (a relative Cartesian co-ordinate)
To point: @12<0 (a relative polar co-ordinate)
To point: @0,-12 (another relative Cartesian co-ordinate)
To point: C (to close)
Try this sequence out and watch the square drawn as you enter each co-ordinate value. You can also use this method to quickly draw a rectangle of known size. Say you needed to draw a rectangle 20 drawing units wide and 10 drawing units high and you didn't mind where exactly the rectangle is drawn, you could do this:

Command Sequence

Command: RECTANGLE
Specify first corner point or [Chamfer/Elevation/Fillet/Thickness/Width]: (pick a point near the middle of the drawing area)
Specify other corner point or [Dimensions]: @20,10
Note that the relative X co-ordinate determines the rectangle width and the relative Y co-ordinate determines the rectangle height.