The act of knocking on a door is usually an indication that the person doing the knocking would like to either talk to someone on the other side or would like to pass through the doorway but cannot due to some form of restriction¹. The act is usually performed by hitting the knuckles of one hand on the door in question one or more times, causing the door to vibrate. This produces sound waves on the other side of the door, alerting those on the other side of the door to your presence². The act has remained popular despite the invention of the doorbell, especially in situations where this wonder of modern technology is not present³.

Reasons to Knock on a Door

Apart from the obvious reasons for knocking on a door, there are several others. The following are times when it is usually appropriate to knock on a door:

• As a way of asking to enter through the door.
• As a way of asking to speak to someone.
• As a way of getting someone to open the door so that an oversized object such as a parcel can be delivered to them⁴.
• To get someone's attention so that you can shout a message to them through the door while not expecting a reply.
• In order to wake someone up in the morning⁵.
• As a way of determining if the person who is going to let you through is actually on the other side of the door or if you're going to be stuck out in the cold for a rather long time.
• As a way of getting the people on the other side to stop making as much noise.
• In order to enquire whether there is someone in the room already, for example in a toilet.
• As part of a futile bid for freedom after trapping a limb in the closing door of a London Underground train.
• As a means of indicating that you would rather like to leave somewhere as a matter of urgency⁶.
• To let someone know that their chimney is on fire.
• To determine whether the door is secure against potential burglars.
• To be generally very irritating. An example of this is knocking on the door and then leaving quickly so that there is nobody around when the door is opened. This is commonly known as 'knock down ginger' and 'knock and run'.
• To determine whether a wooden door is hollow.
• To bring good luck and ward off evil spirits - in European culture the act of knocking on wood is lucky.

Knocking at night is not entirely advisable as it may elicit an angry response, but instead you may just be able to hear the classic line 'Now who can that be at this time of night?' On the other hand, during the day people may knock on several doors, one after the other, making the same request at each⁷. In this case the knocker wishes to talk to the person on the other side of the door, but cares little as to who that person actually is. Well-known practitioners of this form of knocking are door-to-door salesmen, charity workers, Jehovah's Witnesses and Danny Baker (or any of the other men famous for doing the 'Daz Doorstep Challenge' adverts).

Different Ways of Knocking on a Door

It is easy to vary the volume level of the knock and this is often used to illustrate anger or frustration. Knocks may also be increased in intensity if no reply is forthcoming and persons with authority will often waste no time by skipping straight to a fortissimo⁸ knock. It is usual to knock three times then wait before knocking again, although this varies depending on who is doing the knocking and the mood that they are in. Knocks can also be performed to a certain rhythm, sometimes to indicate that a certain person is at the door, and other times just due to the whimsical nature of the person knocking.

Doors can be knocked on using just about any part of the body, but the hand is most commonly used. Having said this, it has been known for people to use their elbows, feet and even their head to knock on doors, despite the risk of damaging the head, the door, or even both. This form of door-knocking usually takes place during periods when both hands are otherwise occupied.

Exterior doors may have knockers which may be used to knock on the door. These are usually lumps of metal attached by a hinge to a metal plate, and are operated by hitting the former on the latter. Famous door knockers include the sanctuary knocker at Durham Cathedral and the door knocker in Dickens' 'A Christmas Carol', which took on the face of Scrooge's dead business partner. The aforementioned hinged implements also caused much controversy on the children's show Blue Peter when presenter Simon Groom led on from a piece about two antique examples with the phrase 'What a lovely pair of knockers.'

Professional door-knockers may use spoons to knock on doors, as repeated use of the hands to knock on doors can lead to soreness. In the case of a glass door, a coin can be used by placing it between the thumb and the middle finger and putting the tip of the index finger firmly behind it to add force. In some parts of Africa where there are no doors it is common to clap instead of knocking.

Unexpected Dangers of Knocking on Doors

As with most things, there are dangers attached to the act of knocking on doors. The most common include being set upon by large dogs, discovering that the door has a large number of splinters, being attacked by an unhappy owner and wet paint. Many other unexpected things may happen, and readers are warned to remain vigilant at all times when attempting to knock on doors. Wearing a good pair of running shoes may also be advisable.

Door-knocking in Popular Culture

There are many instances of door-knocking in books, plays, films and music. Most children are familiar with the tale of the Three Little Pigs where the Big Bad Wolf knocks on the door of their house, but is refused entry and is forced to blow each house down⁹. Although horror films often use knocking to create suspense, no-one took knocking on doors to such an extreme as Jack Nicholson in 'The Shining' when he chopped through a door using an axe with the line 'Here's Johnny!' Knocking also features in the lyrics of many songs, including 'Who Can It Be Now' by Men at Work and 'Knockin' on Heaven's Door' by Bob Dylan.