The 900 series allows the user to plot general titles up to 72 characters in length, chemical symbols up to 6 characters long, and bond length labels. The bond length labels can have two decimal places before the decimal point and one, two, or three places after the decimal point. The 700 and 800 series instructions can plot chemical symbols and bond length labels, but it is often desirable to position certain labels individually with the 900 series.

ORTEP-III provides the following character set for labels:

General titles and bond length labels can be drawn either along a general vector baseline or parallel to the plane of the drawing. Chemical symbols are always drawn parallel to the plane. Instructions 913-916 produce general plane lettering, and instructions 901-906 produce normal plane lettering.

Two vectors, the upright vector and the base-line vector, are needed to describe a lettering plane. In ORTEP the upright lettering vector is always parallel to the plane of the drawing. For perspective lettering the base-line vector is a general vector in three dimensions. In the nonperspective case, the base-line vector is either along the projection of a general vector or along the vector (in the plane of the drawing) that is oriented with a 302 title rotation instruction (theta base line). If theta is zero, then the theta base-line vector is along the plotter positive x axis.

The exact center of the label is always referred to when specifying the position of the label. The program goes through the following steps to position the center point of the label onto the drawing.

(1) A point P1 is found that is either the position of atom A (columns 10-18) or the mean of two atom positions (atom A and atom B) if an atom designator exists in the atom B field (columns 19-27) of the instruction card.

(2) A point P2 is found by (a) translating from P1 along the base-line vector for the distance specified by the parallel offset, then (b) translating along the upright vector by the perpendicular offset distance.

(3) A point P3 is found by projecting P2 onto the plane of the drawing.

(4) If the "x edge reset" is greater than 0, then x is reset to this value. If "x edge reset" is less than 0, x is reset to the positive x plot boundary minus |x edge reset|. No resetting is done if x edge reset equals 0. The y parameter is handled in the same manner with "y edge reset".

Instruction 901 produces a normal plane atom label with theta base line, using the atom label for atom A.

A normal plane title with theta base line is drawn with the 902 instruction. The title must be provided with the instruction on a Format No. 3 trailer card. The title should be centered about columns 36-37 of that card. However, if the title begins in column 1, it will be centered within the field automatically. (New feature in ORTEP-III)

A general vector title with normal plane lettering is produced by the 903 instruction and with general plane lettering by 913. The general vector is from atom A to atom B. The title must be provided with the instruction on a Format No. 3 trailer card. The title should be centered about columns 36-37 of that card. If the title begins in column 1, it will be centered automatically.

Instructions 904, 905, 906, 914, 915, and 916 produce general-vector bond-length labels. The first three are for normal plane lettering with one, two, and three places after the decimal point, respectively; and the last three are for the corresponding bond-length labels with general plane lettering. The general vector is from atom A to atom B. Note that the sense of the vector is important in order to have the label right side up.

The format for the entire 900 series is as follows:

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Page last revised: March 5, 1996