Rebalance 2D Classes is used as a means of addressing particle datasets that are highly imbalanced in terms of orientation sampling. This issue is known as preferred particle orientation, which can occur when particles tend to "stick" to the air-water interface. One effect of strong orientation bias that can be observed in 3D maps is "smearing" of the overall map along a particular axis. Generally it is only recommended to use this job after an initial Ab-initio Reconstruction job has already been done, and erroneous features have been observed, as datasets with moderate orientation bias often still produce reasonable quality Ab-initio reconstructions. However, for some datasets, it has been observed that simply discarding particles from the oversampled views can be useful in addressing preferred orientation, which is the main functionality of this job.