Xy-pic User's Guide 
2.5 More targets
The target address can be given in a large number of formats called positions. The full range of possibilities is described in the reference manual [8, fig. 1,]; here is a number of useful ones in addition to the hop format described in section 1.3:
??!C[], where r,c are integers, denotes the
relative entry found r rows below and
c columns to the right of the current entry (the current entry itself is thus [0,0]). This always corresponds to a[hop], e.g.,[1,2]is the same as[drr]and[-2,0]is the same as[uu].
, where r,c are positive integers, denotes the
absolute entry found in the rth row and cth column
of the diagram. The top left entry is "1,1".
, where t' is any target, changes the base entry
of the present arrow to t' and then sets the target to t relative
to the original base entry. For example,
typesets\xymatrix{ A \\ B & C \ar@<1ex>[ul] \ar@<1ex>[ul];[] }
i.e., the second
\ar[ul]arrow starts at the[ul]entry and ends in the current entry.
Xy-pic User's Guide
© Kristoffer H. Rose <krisrose@brics.dk> January 6, 1997 / LaTeX2HTMLv96.1-h