The JKD 1-2 Series refers to training to defend against the Jab-Cross combination. This is a fundamental training series in the JKD/Jun Fan curriculum and should be trained in the left lead, right lead, and the mixed lead (left vs. right and right vs. left). A basic understanding of the various lines the Cross can come in on will make this training more beneficial so pay particular attention to which counters match up to a wide cross and which match up to a tight cross - they are not necessarily interchangeable!
