Recombination Signal Sequences
The regional genes (V, D, J) are flanked by Recombination Signal Sequences (RSSs) that are recognized by a group of enzymes known collectively as the VDJ recombinase.
RSSs are composed of seven conserved nucleotides (a heptamer) that reside next to the gene encoding sequence followed by a spacer (containing either 12 or 23 unconserved nucleotides) followed by a conserved nonamer (9 base pairs).
The RSSs are present on the 3' side (downstream) of a V region and the 5' side (upstream) of the J region.
These are the sides that will be involved in the joining. Only a pair of dissimilar spacer RSSs are efficiently recombined (i.e. one with a spacer of 12 nucleotides will be recombined with one that has a spacer containing 23 nucleotides).
This is known as the 12/23 rule of recombination.