@<>
s: it specifies how far `sideways' the arrow
should be moved, e.g.,
will typeset\xymatrix{ A \ar@<1ex>[dr]^a_{.} \\ & B \ar@<1ex>[ul]^b \ar@<1ex>[r]^c & C \ar@<1ex>[l]^d_{.} }
A positive distance will slide the arrow in the `^
-direction', e.g.,
the two arrows above are slid in the direction of the labels a,
b, c, and d, respectively; a negative distance in the
`_
-direction'. The distance <@1ex>
is often appropriate since it
corresponds roughly to the height of letters like `x', in the type size
being used.