Constructional apraxia is characterized by difficulty or loss of the ability to build, assemble, or draw objects. It is a neurological disorder in which a person is unable to perform a task or movement despite understanding it, having the will to complete it, and having the physical ability to do so. Constructional apraxia may result from parietal lobe lesions following stroke or may be a sign of Alzheimer's disease. The inability to copy or draw a picture correctly is a core disability in patients with constructional apraxia. The disorder shows qualitative differences between patients with left hemisphere damage, right hemisphere damage, and Alzheimer's disease. Patients with left hemisphere damage tend to memorize elements, oversimplify the features of a drawing, and neglect details when drawing from memory. In addition, patients with left hemisphere damage are less likely to be able to arrange the parts of their drawing systematically. Patients with right hemisphere damage have difficulty correctly reproducing the spatial relationships of complex shapes. Drawing elements are often scattered, moved to different locations or directions, or are diagonal on the page. As a result, patients with right-hemisphere damage tend to produce distorted or asymmetric drawings characterized by hemispatial neglect, that is, neglect of elements on one side of the model. Patients with right-hemisphere damage were previously thought to be twice as likely to make errors on 3D construction tasks as those with left-hemisphere damage. This inaccurate finding was attributed to a selection bias, as researchers excluded people with severe left-hemisphere lesions because they had debilitating language impairment. However, studies included people with severe right-hemisphere lesions. Subsequent research has provided evidence that there is no significant difference in performance on 3D construction tasks between right-hemisphere and left-hemisphere patients. Alzheimer's patients with constructional apraxia have unique symptoms. Drawings contain reduced angles, spatial distortions, lack of perspective, and numerous simplifications, which are uncommon in patients with both left and right hemispheric damage. Developmental disturbances appear early in the disease and worsen over time; however, patients with advanced Alzheimer's disease may be able to perform some developmental tasks. Automatic drawing is affected early on, as it relies heavily on semantic memory; simplifications in drawing may therefore occur as a result of impaired access to semantic knowledge. As Alzheimer's disease progresses, the patient's ability to copy objects deteriorates and the patient is more likely to be unable to draw figures correctly due to motor loss of routine memories. In 1934, Carl Kleist characterized developmental apraxia as a disorder "of formative activities such as assembling, building, and drawing, in which the spatial form of the products is impaired, without apraxia of the individual movements." Over the following years, the definition of developmental apraxia has diversified. Some have considered it an executive processing command, while others have seen it as a visual-spatial disorder. As a result of the conflicting definitions, apraxia of construction has become an umbrella term to describe any type of constructional disorder. Contemporary researchers remain skeptical about the appropriateness of the term "apraxia" to describe this disorder.