Reciprocal apprenticeship was a form of mutual apprenticeship between Jedi. A facet of the Concordance of Fealty, a ceremonial expression of trust by the exchange of the lightsabers of two Jedi, it had become rarely-practiced following the Cleansing of the Nine Houses in the Old Sith Wars conflicts.
To complete her training in the Force after abandoning its dark side, the former Emperor's Hand Mara Jade sought out Jedi Knight Kyle Katarn and undertook a reciprocal apprenticeship with him, creating a bond between the two that allowed Jade to redeem Katarn from the dark side of the Force in 10 ABY.