# Gradient Boosting: Is it possible to use a weak classifier?

My understanding is that a regressor has to be used to fit to the residual. Is it possible to directly apply a classifier? If so, what are the requirements/restrictions?

It is possible to use classifiers, though. AdaBoost, which is the special case Gradient Boosting was derived from, uses $\{-1,1\}$ classifiers. Simplified, AdaBoost learns a simple classifier, then checks which points it got wrong, and learns a new classifier that puts more weight on the ones that were wrong before.