# Basic Concept of Operation

When you have successfully installed and started GrADS, you'll be confronted with two windows -- a terminal window with a prompt, much like the infamous C:> in MS-DOS, and a resizable window (black background by default) where graphics are displayed.

GrADS commands are entered in the terminal window and the response from GrADS is either graphics in the graphics window or text in the terminal window. The three fundamental GrADS commands:

1. `open` open or make available to GrADS a data file with either gridded or station data
2. `d` display a GrADS "expression" (e.g., a slice of data)
3. `set` manipulate the "what" "where" and "how" of data display

The GrADS "expression," or what you want to look at, can be as simple as a variable in the data file that was opened, e.g., `d slp` or an arithmetic or GrADS function operation on the data, e.g., `d slp/100` or `d mag(u,v)` where mag is a GrADS intrinsic function.

The "where" of data display is called the "dimension environment" and defines which part, chunk or "hyperslab" of the 4-D geophysical space (lon,lat,level,time) is displayed. The dimension environment is manipulated through the set command and is controlled in either grid coordinates (x,y,z,t or indices) or world coordinates (`lon, lat,lev, time`).

The "what" and "how" of display is controlled by `set commands` and includes both graphics methods (e.g., contours, streamlines) and data (e.g., `d` to a file).

GrADS graphics can be written to a file (i.e., `enable print` filename and `print`) and then converted to postscript for printing and/or conversion to other image formats.

In addition, GrADS includes graphic primitives (e.g., lines and circles) and basic labelling through the `draw command`.

The `q` or `query` command is used to get information from GrADS such as which files are opened and even statistics.