‘Normal’ comes from the latin norma, a carpenter's square, and in mediaeval times meant something at a right angle or perpendicular. A few centuries later it came to mean conforming to a standard and was used in physics to refer to the average of a set of variables. If the variable concerned, such as head size or IQ, is continuous and has a normal distribution, by convention the normal range has come to include values falling within two standard deviations either side of the mean – defining about 4.6% of the population as abnormal. By a similar approach, if the variable has a skewed distribution, such as the age of walking, those lying outside the 2nd and 98th centiles are often defined as abnormal, while the normal is equated to the average. This can also lead to misunderstandings, as, for example, in some child-rearing books when parents are told that the normal age of walking is 12 months, which can mistakenly imply that the 50% of children, who by definition are not walking at that age, are abnormal.