The words vowel and consonant are very familiar ones, but when we study the sounds of speech scientifically we find that it is not easy to define exactly what they mean. The most common view is that vowels are sounds in which there is no obstruction to the flow of air as it passes from the larynx to the lips. A doctor who wants to look at the back of a patient's mouth often asks the patient to say "ah"; making this vowel sound is the best way of presenting an unobstructed view. But if we make a sound like s or d it can be clearly felt that we are making it difficult or impossible for the air to pass through the mouth.