Understanding Commands
In its simplest form, a Command is how you tell AutoCad what you want it to do. AutoCad's complete functionality is based on the commands that are inputted by the user. All other aspects of AutoCad (system variables, options, toolbars and menus) revolve around what the commands will look/feel like. Most commands, when you enter commands , will return either a set of options or a dialog box. For example, when you enter Circle at the Command prompt, the following prompt is displayed:
Specify center point for circle or [3P/2P/Ttr (tan, tan, radius)]:
Inputting Commands
There are 4 ways of inputting commands: command line, toolbar, menu and/or ribbon. Let's look at an example: I have opened a new session of AutoCad. Upon opening, the drawing itself is a clean slate:
Menu Bar
At the top of the display are the menu's (File, Edit, etc). The classic menu bar is an older feature of AutoCad and is displayed at the top of the drawing area. If you click on one of the menu, there is a drop down list of commands:
If you wanted to draw a line, simply click on the "Draw" menu and then click on "Line":

Command Line
Another way of enterring a command is by typing it in at the command line. For example, if I want to draw a line, I'll simply enter the "line" command at the command line and then press Enter or the Spacebar:

Toolbar
Commands can also be entered using a toolbar. The first step here is to make sure that the toolbar that has the coomand you'll use is visible in the drawing area. For example, the "Line" command resides on the "Draw" toolbar so click on the Line icon to initiate the command:

Ribbon
Finally, the newest way to enter a command is using the Ribbon panels which are organized by rows, sub-panels, and slideout elements. Rows and sub-panels are used to organize how commands and controls are displayed on the ribbon panel. A slideout is automatically added to each ribbon panel and controls which rows are shown by default. Rows located below a slideout are displayed only when a ribbon panel is expanded. So keeping with the same "Line" command, let's show the ribbon panel looks when enetering the Line command:

To learn how Options affect the AutoCad user click here. Or if you prefer, learn about the Essential Commands.
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