Subfield $w in Cross-References

Control Subfield - $w

The $w is a control subfield with up to four character positions. One-character alphabetic codes are used to describe special instructions that apply to the display of 4xx and 5xx fields. The definitions of the codes in subfield $w are dependent on the character position in which they occur, the coding of any character position mandates that each preceding character position contains a code or a fill character (|); succeeding character positions need not be used. For example, if $w/0 and $w/1 are not applicable but $w/2 is applicable, then $w/0 and $w/1 each contain code n (Not applicable) or a fill character (|).  The n is used more often than the fill character. 
The positions available with the subfield $w are as follows:

Position /0 = Special relationship

Position /1 = Tracing use restriction

Position /2 = Earlier form of heading

Position /3 = Reference display

Each position has a set of codes that further determine the meaning of that position. As described above if the position is not used an "n" meaning not applicable is used.

Lets attempt to create a $w code. Details on what each code for every position means can be found at this site; http://www.loc.gov/marc/authority/adtracing.html under MARC21 Authority Format the Tracings and References-General Information section.

For this example, the heading 400 1 Callaghan, Bede Bertrand,$cSir,$d1912- is an earlier Pre-AACR2 form of Callaghan, Bede,$cSir,$d1912-

We want this heading as a see reference but there are two things we want to indicate with the $w. We want to let the user know that this is a pre-AACR2 heading (Position/2, code a) and we want to make sure this does not display as a see reference in the reference module. (Position/3,code a) With this in mind, our $w will be coded as follows.

400 1 $wnnaaCallaghan, Bede Bertrand,$cSir,$d1912-