A Death Knell was the ringing of a bell immediately after a death to announce it. Historically it was the second of three bells rung around death; the first being the "Passing Bell" to warn of impending death, and the last was the "Lych Bell", or "Corpse Bell", which survives today as the Funeral toll.
In England, an ancient custom was the ringing of bells at three specific points before and after death. Sometimes a "passing bell" was first rung when the person was still dying. then a bell at the actual time of death - the Death Knell, and finally the "Lych Bell" which was rung at the funeral as the procession approached the church. .
The Death Knell was regulated by statutes in the time of Henry VIII and Elizabeth I.  but the immediate ringing after death fell into disuse. It was customary by the end of the 19th century to ring the Death Knell as soon as notice reached the clerk of the church (parish clerk) or sexton, unless the sun had set, in which case it was rung at an early hour the following morning. 
Manner of ringing
The manner of ringing the Knell varied in different parishes. Sometimes the age of the departed was signified by the number of chimes (or strokes) of the bell, but the use of "tellers" to denote the sex was almost universal, and by far the greater number of churches in the counties of Kent and Surrey used the customary number of tellers, viz., three times three strokes for a man and three times two for a woman, with a varying use for children across the counties. The word "tellers" became changed into "Tailors".  J. C. L. Stahlschmidt produced comprehensive lists of the practices at each church in Kent and Surrey in his two volumes of the bells of those counties.
The use of the passing bell for sick persons is indicated in the Advertisements of Queen Elizabeth I issued in 1564... "where any Christian bodie is in passing, that the bell be tolled, and that the curate be specially called for to comfort the sick person" 
A more modern tradition at funerals where there are full circle ring of bells is to use "half-muffles" when sounding one bell as a tolled bell, or to ring all the bells in change ringing. Half-muffling means a leather muffle is placed on the clapper of each bell so that there is a loud "open" strike followed by a muffled strike, which has a very sonorous and mournful effect. An excellent example of this was demonstrated with the bells of Westminster Abbey at the Funeral of Diana, Princess of Wales in 1997.
- Walters P156. He also notes " ..there was sometimes the inconvenience that though the Passing Bell had been duly rung, the dying person might recover"
- Timbs, John (1863). Mysteries of Life, Death, and Futurity: illustrated from the best and latest authorities. New York.
- Brand, John; Ellis, Sir Henry; Halliwell-Phillipps, James Orchard (1849-01-01). Observations on the Popular Antiquities of Great Britain: chiefly illustrating the origin of our vulgar and provincial customs, ceremonies, and superstitions. Bohn.
- Walters P 160
- "The passing bell". Things Not Generally Known; Familiarly Explained. Lockwood & Company. 1867.
- "Correspondence - answers: The passing bell". The Churchman's Companion. 1868.
- E.g. Thomas Hood's poem "Faithless Sally Brown"; "His death which happened in his berth, At forty-odd befell: They went and told the sexton, and The sexton tolled the bell"
- Walters P157
- Walters P160
- Walters P157-158
- Stahlschmidt J. C. L.: The Church Bells of Kent: Their inscriptions, founders, uses and traditions, p. 126. London: Elliot Stock, 1887.
- Stahlschmidt, J. C. L.: Surrey Bells and London Bell Founders: A Contribution to the Comparative Study of Bell Inscriptions, p. 124. London: Elliot Stock, 1884.
- Walters P155
- Walters P 160
- H B Walters, The Church bells of England. published 1912 and republished 1977 by Oxford University Press.