Keywords are fields within an ARD description which specify one of the basic shapes known to ARD. Most are followed by an argument list giving values for the size, position, orientation, etc., of the shape. Argument lists are contained within parentheses, and arguments are separated by commas. Positions and distances should be supplied in a format appropriate to the current user coordinate system as determined by the WCS and/or COFRAME statements. Keywords can be abbreviated to three characters. The following keywords are currently supported ($N$ represents the dimensionality of the ARD description, and all positions and distances are given in $N$-dimensional user coordinates):

- BOX
- - A rectangular box with sides parallel to the user coordinate axes. The argument list should contain $2\ast N$ values; the first $N$ values give the coordinates of the box centre, and the remaining $N$ values give the lengths of the box sides. It may sometimes be more convenient to use the RECT keyword which specifies a rectangular box in terms of two diagonally opposite corners.
- CIRCLE
- - A circle (for $N=2$) or sphere (for $N>2$). The argument list should contain $N+1$ values; the first $N$ values give the coordinates of the centre of the circle or sphere, and the remaining value gives the radius (in user coordinates).
- COLUMN
- - A set of lines parallel to the second axis of the user coordinate system. The argument list can contain any number of values (one for each of the lines). Each argument value $X$ specifies that the corresponding line should pass through the position $\left(X,0\right)$. This keyword can only be used in 2-dimensional ARD descriptions.
- ELLIPSE
- - A 2-dimensional ellipse. The argument list should contain 5 values; the first pair give the user coordinates of the centre of the ellipse, the second pair give the half-lengths of the two axes of the ellipse (in user coordinates), and the fifth value gives the angle (in degrees) from the first axis of the user coordinate system to the first axis of the ellipse (positive rotation is from the first to the second user axis). This keyword can only be used in 2-dimensional ARD descriptions.
- FRAME
- - The entire mask excluding a border of given width. The argument list should contain a single value giving the width of the border. This keyword can only be used in 2-dimensional ARD descriptions. A further restriction on its use is that the current mapping from user coordinates to pixel coordinates must be isomorphic (i.e. each mask pixel must correspond to a square area in user coordinates; the square may be rotated, shifted and/or scaled, but it must still be a square). If this is not the case an error will be reported by ARD_WORK.
- INPUT
- - The pixel mask supplied as input to routine ARD_WORK.
- LINE
- - A straight line between two given positions. The argument list should contain $2\ast N$ values, the first set of $N$ values giving the user coordinates of the first position, and the second set of $N$ values giving the second position. Only the section of the line between (and including) the two positions is included in the mask.
- PIXEL
- - A set of individual pixels. The argument list should contain an integer multiple of $N$ values; each set of $N$ values giving the user coordinates of a point to be included.
- POINT
- - POINT is a synonym for PIXEL.
- POLYGON
- - A 2-dimensional polygonal area. The argument list should contain an even
number of values; each pair giving the user coordinates of a vertex of the polygon. These
vertices are joined together in the order given to form the polygon. The last vertex is
joined to the first to close the polygon. This keyword can only be used in 2-dimensional
ARD descriptions. Note, the edges of a polygon are
*geodesics*within the user coordinate Frame. So, for instance, if user coordinates are RA and DEC then the edges of a polygon will correspond to great circles on the sky. - RECT
- - A rectangular box with sides parallel to the user coordinate axes. The argument list should contain $2\ast N$ values; each set of $N$ values giving the coordinates of a pair of diagonally opposite corners. It may sometimes be more convenient to use the BOX keyword which specifies a rectangular box in terms of its centre and dimensions.
- ROTBOX
- - A rotated box. The argument list should contain 5 values; the first pair give the
coordinates of the box centre, the second pair give the lengths of the two sides, and the
fifth value gives the angle (in degrees) from the first axis of the user coordinate system
to the first side of the box (positive rotation is from the first to the second user axis). This
keyword can only be used in 2-dimensional ARD Note, the edges of a rotated box are
*geodesics*within the user coordinate Frame. So, for instance, if user coordinates are RA and DEC then the edges of a rotated box will correspond to great circles on the sky. - ROW
- - A set of lines parallel to the first user axis. The argument list can contain any number of values (one for each of the lines). Each argument value $Y$ specifies that the corresponding line should pass through the position $\left(0,Y\right)$. This keyword can only be used in 2-dimensional ARD descriptions.
- WHOLE
- - This keyword selects all pixels in the mask. It has no argument list.