Understanding How Command Options Work

Nearly every AutoCAD command offers a set of options that are shown at the command-line prompt. These options let you alter the behavior of a command to suit your current drawing. To see how command options work and to get a feel for the drawing process in general, in this exercise you'll draw an arc and then place it exactly in the inside corner of the rectangle:

1. Click the Arc tool in the 2D Draw control panel.

   The prompt Specify start point of arc or [Center]: appears, and the cursor changes to a crosshair cursor.

   

2. If you examine this Specify start point of arc or [Center]: prompt, you'll see the start point contains two options. The default option is stated in the main part of the prompt: Specify start point. If other options are available, they appear within brackets, as in the [Center] option that appears in the Arc Command prompt. This [Center] option tells you that you can also start your arc by selecting a center point instead of a start point. If multiple options are available, they appear within the brackets and are separated by slashes (/). The default is the option AutoCAD assumes you intend to use unless you tell it otherwise.

3. Enter C↲ to select the Center option. The prompt Specify center point of arc: appears. Notice that you had to enter only the C and not the entire word Center.

   When you see a set of options in the command line, note their capitalization. If you choose to respond to prompts using the keyboard, these capitalized letters are all you need to enter to select that option. In some cases, the first two letters are capitalized to differentiate two options that begin with the same letter, such as LAyer and LType.

4. Now select a point for the center of the arc, as shown in Figure 1.12. The prompt Specify start point of arc: appears. You'll also see a rubber-banding line from the center point you just selected to your cursor.

   If you point directly to the right, you'll see that the rubber-banding line snaps to an exact horizontal orientation, and you'll see a ToolTip appear at the cursor. This is a feature called polar tracking vector, and it helps you to draw in exact horizontal and vertical directions much like a T square and a triangle. The ToolTip shows your cursor's location relative to the center point you just selected. It displays this information in what is known as a polar coordinate. You can learn more about polar coordinates in Chapter 2.

5. With the rubber-banding line pointing to the right, click to select a point, as shown in Figure 1.12. The prompt Specify end point of arc or [Angle/chord Length]: appears.

6. Move the mouse, and a temporary arc appears, originating from the start point of the arc that you just selected and rotating about the center of the arc.

   As the prompt indicates, you now have three options. You can enter an angle, a chord length, or the endpoint of the arc. The prompt default, to specify the endpoint of the arc, lets you select the arc's endpoint. The cursor is in Point Selection mode, telling you it is waiting for point input. To select this default option, you need only select a point on the screen indicating where you want the endpoint.

7. Move the cursor so it points vertically from the center of the arc. You'll see the polar tracking vector snap to a vertical position, as shown in Figure 1.12.

8. Click any location with the polar tracking vector in the vertical position. The arc is now fixed in place.

Figure 1.12: Using the Arc command

As you can see, AutoCAD has a distinct structure in its prompt messages. You first issue a command, which in turn presents options in the form of a prompt. Depending on the option you select, you get another set of options or you are prompted to take some action, such as selecting a point, selecting objects, or entering a value. The prompts offer a great deal of help "prompting" you to take an action.