Control of Error
Montessori materials are designed in a way that allow the child to know whether or not he has done the work correctly. In this way, the child does not need to rely on the teacher to correct his work, but instead, fix any mistakes he has made himself. The term "Control of Error" is used to describe the intrinsic properties of the work that allow this.
Some Montessori materials do not have intrinsic Control of Error and for these the child must rely on the teacher or an older child for correction.