In partially linear systems, such as the Lorenz model, chaotic synchronizat
ion is possible in only some of the variables. We show that, for the nonsyn
chronizing variable, synchronization up to a scale factor is possible. We e
xplain the mechanism for this projective form of chaotic synchronization in
three-dimensional systems. Projective synchronization is illustrated for t
he Lorenz and disk dynamo systems. We also introduce a vector field that ca
n be used to predict the scaling factor. [S0031-9007(99)08897-3].