In 1974, Teasdale and Jennett published a paper in the Lancet, entitled "Assessment of coma and impaired consciousness: a practical scale". This provided a schema for the assessment of graded responses in three behavioural domains - opening of the eyes, the capacity to communicate verbally and a motor response to move parts of the body after a command or physical stimulus. A patient suffering from an acute brain injury is assessed by their response in each test. If one test is not available, for example because of swelling around the eyes or breathing difficulties, the others are still available.

The effectiveness and ease of use is shown in that the GCS soon became utilised world-wide in Neurosurgical Units and in Accident and Emergency departments. It has been translated into more than 60 languages and the 1974 paper is the most highly cited in clinical neurosurgery. Numbers assigned to the steps in the responses can be summated into a total score. The maximum score attainable by a patient is 15, the minimum score is 3. The summated value has a strong correlation with the outcome for a patient and is used in research and in classifying patients. A total score between 3 and 8 indicates a severe injury to the brain which will have a poor prognosis, between 8 and 12 a moderate injury and between 13 and 15 a mild injury. The great majority, more than 85%, of patients admitted to emergency triage appear to have mild head injury and of these about half will make a full recovery.