The use of seals goes back to at least 4000 BCE, rendering the practice of sealing even older than writing. Initially the seal embodies the representation, and even personification, of a person; a function comparable to that of the signature. Seals’ materials differ in each time period: There are golden seals (imperial seals, since the fourth century), lead seals (since the eighth century), wax seals (since the medieval period), cylinder seals (since the fifteenth century), wafer seals (seventeenth and eighteenth century), embossed paper seals (seventeenth and eighteenth century), and metal seals (gilded, seventeenth and eighteenth century). The manner in which seals are attached to a document (applied, pendant, adherent, or hanging seals) provides information about the type of document and the time period in which the document has been drafted.
Seals are roughly divided into four categories: royal and semi-royal (great seal, double-sided seals, secret seal, signet), ecclesiastical, local, and personal or private seals. As each category of seals has its own particular function, it is important to remember that it is not unusual for one single person to use various different seals. Images on seals can be divided into the following categories: head or bust, full-length, seated, equestrian seals, depictions of characters performing an activity, symbolic imagery, heraldic seals and cut stones.
Seals’ primary function is authentication and because of this seals have an important legal function, particularly in the medieval period. The secondary use is the sealing and closing of documents as well as objects (up until the invention of the envelope with gum rim in the nineteenth century). The seal design’s style is time- and fashion-dependent. Seal owners are not only found among the elite but also among the middle class (clerks, traders, and merchants, among others). Beside of a legend, a seal often depicts family crests and mytholigical animals. Offried Neubecker writes:
Wapens vertegenwoordigen personen of groepen van personen alsof ze het zelf waren. De aanwezigheid van een wapen fungeert als plaatsvervanger van de persoon, zelfs na zijn dood. Daarom zijn wapens ook geliefde emblemen op zegels en stempels; ze mogen echter daarmee niet volledig op één lijn worden gesteld.
[Family crests represent persons or groups of persons as if it were they themselves. The ]presence of a crest functions as the substitution for the person, even after this person’s death. This is why family crests are popular emblems on seals and stamps, but they should not be fully equated with one another.
Seals are fragile, which already led a variety of protective measures during the medieval period. Three commonly known methods to protect the seal are the folding of the parchment and the use of seal-bags and skippets.
The personal seal as a historical source
Personal seals were used by individuals for private affairs and provide unique insights into the sealer’s personality and social status, particularly if the sealer is a woman. Elizabeth New argues that
[r]ecent work has highlighted the importance of seals in studying medieval women […]. In some cases seals gave medieval women the opportunity to identify themselves in a way other than as wife, daughter or mother, the key ways they were recorded in documents. […] Seals however were generally the choice of their owner, and so provide a brief glimpse into the ways in which people saw themselves or wanted to be seen, and often into individual piety, family or social concerns, or humour.
[I]t is always important to remember when looking at a personal seal that it belonged to someone and was (usually) their choice, something they may have carried with them, and was perhaps a keepsake or one of their few private possessions.
In contrast with virtually all other non-written sources, we know of most seals when, where, for what reason, and by whom they were used. It must be noted, however, that due to seal matrixes’ longevity these were often passed on to next generations.
Seals provide vital information for virtually all historical disciplines: diplomatics (the study of charters), philology, epigraphy (the study of inscriptions) and paleography, archaeology, genealography and social history, political, legal, and administrative history, anthropology, and cultural history. Furthermore, seals are one of the oldest sources containing heraldic imagery and therefore very suitable studying the development of family crests in general or that of specific families. Images on seals are thus an important source of information for art historians. Furhtermore, a clear connection can be discerned between the design of coins and seals, particularly in the medieval period, which has not yet been futher studied.
A greatly limiting factor in the study of seals is that seals are made from fragile materials that are easily damaged. While it is supposed that a personal seal is unique, as it provides documents with an unmistakable authenticity and validation, there exist so-called ‘anonymous’ seals as well, without a name in the legend (its design identical to ‘real’ seals) which were produced and sold in great quantities. This practice, as well as the many forgeries that are in circulation, complicate the correct identification of seals (and sealers). Generally the biggest challenges for sigillography are the accessibility and raising awareness to the fact that seals are crucial sources for all types of research of the past. Although much work is done to set up a digital database for zegels, the arrival of one single all-encompassing global digital database for seals still seems to be far away. In the specific case of personal seals research is hindered by the fact that this type of seal can be found on all different types of and can thus be found in all different types of archives. But despite of all the limitations, sigillography is a fascinating discipline that is definitely worth of studying (and very rewarding too, if I may add).
 “Het gebruik van het zegel wordt in het vierde millenium voor onze tijdrekening reeds vermeld in Mesopotamië, dus nog voor het schrift verschijnt” [The use of the seal is already mentioned in Mesapotamia in the fourth millenium BCE, so even before the first appearance of the written word] (E. Kittel, Siegel, 7-115, cited in René Laurent, Sigillografie (Brussel: Algemeen Rijksarchief, 1986), 5.
 Brigitte Miriam Bedos-Rezak, “Medieval Identity: A Sign and a Concept,” The American Historical Review 105, no. 5 (2000), 1489-1533, aldaar 1489-1490. Hilary Jenkinson, A Guide to Seals in the Public Records Office (London: Her Majesty’s Stationary Office, 1968), 4-5.
 Ted Steemers, Zegels: geschiedenis, gebruik, conservering, restauratie (Maastricht: Rijksarchief in Limburg, 1984), 63; Jenkinson, A Guide to Seals , 12-13.
 Steemers, Zegels, 38.
 Steemers, Zegels, 38; Jenkinson, A Guide to Seals, 14-20; Elizabeth A. New, Seals and Sealing Practices, vol. 11, Archives and the User (London: British Records Association, 2010), 3; Laurent, Sigillografie, 5; Wilhelm Ewald, Siegelkunde (München: Oldenbourg XIV, 1975), 99-104 identifies two additional categories: “das sigillum publicum (authenticum) und das sigillum secretum”.
 Georges Tessier, Diplomatique Royale Française (Paris: Éditions A. et J. Picard et Cie., 1962), 192-206.
 Based on H.S. Kingsford, cited in Jenkinson, A Guide to Seals.
 Jenkinson, A Guide to Seals, 7-8; Michel Pastoureau, Les Sceaux, vol. 36, Typologie des Sources du Moyen Âge Occidental (Turnhout, Belgium: Brepols, 1981); Jenkinson, A Guide to Seals in the Public Records Office. 41-42.
 Laurent, Sigillografie, 57-61; Ewald, Siegelkunde, 182: “Schrift-Siegel, Bild-Siegel, Porträt-Siegel, Wappen-Siegel”. Idem in Pastoureau, Les Sceaux, 59-61.
 New, Seals, 7,11; Jenkinson, A Guide to Seals, 4; Laurent, Sigillografie. 5; Ewald, Siegelkunde, 24-25.
 François Eygun, Sigillographie du Poitou jusque’en 1515: étude d’histoire provinciale sur les institutions, les arts et la civilisation d’après les sceaux (Mâcon: Protat Frères, 1938), 63-91; New, Seals, 26; Pastoureau, Les Sceaux, 29-30.
 Jenkinson, A Guide to Seals, 5; New, Seals, 11. 7; Pastoureau, Les Sceaux, 21-22; Laurent, Sigillografie. 5: “The seal has always had three purposes: closing (and guarding a text’s contents or secret), validating ownership, and certifying a deed (Y. Metman, “Sigillographie et marques postales.” L’ Histoire et ses méthodes, Parijs, 1964: 393). Not all three of these purposes were always present.”
 Jenkinson, A Guide to Seals, 5; New, Seals and Sealing Practices, 112.
 Jenkinson, A Guide to Seals, 22-29.
 Zie bijvoorbeeld Tessier, Diplomatique Royale Française.
 Jenkinson, A Guide to Seals, 6-7; Pastoureau, Les Sceaux, 28-29; John Alexander McEwan, “The Challenge of the Visual: Making Medieval Seals Accessible in the Digital Age,” Journal of Documentation 71, no. 5 (2015), 999-1028, aldaar 999; New, Seals and Sealing Practices, 28.
 Jenkinson, A Guide to Seals in the Public Records Office. 21-24; New, Seals and Sealing Practices, 11.121.; P.D.A. Harvey, “This Is a Seal,” in Seals and Their Context in the Middle Ages, ed. Phillipp R. Schofield (Haverton, PA: Oxbow Books, 2015),1-5.
 Ottfried Neubecker, Heraldiek: bronnen, symbolen en betekenis (Amsterdam: Elsevier, 1977), 90, 94-95, 97, 187-190, 194, 196.
 Ibid., 7
 New, Seals, 23-25; Jenkinson, A Guide to Seals, 18.
 New, Seals, 23-25.
 Bedos-Rezak, “Medieval Identity,” 1532; New, Seals, 87.
New, Seals, 28.
 New, Seals, 87.
 Ibid., 26.
 Jenkinson, A Guide to Seals, 7.
 Pastoureau, Les Sceaux, 64; Eygun, Sigillographie du Poitou, 36-55; Tessier, Diplomatique Royale Française.
 Pastoureau, Les Sceaux, 66-67.
 Eygun, Sigillographie du Poitou, 56-62.; Pastoureau, Les Sceaux, 66-67.
 Eygun, Sigillographie du Poitou, 113-1131.; Pastoureau, Les Sceaux, 70-72.
 Eygun, Sigillographie du Poitou, 149-154.; Pastoureau, Les Sceaux, 68. New, Seals, 27.
 Pastoureau, Les Sceaux, 64-66. New, Seals, 27.
 Pastoureau, Les Sceaux, 72-74.
 The relation between seals and heraldry is discussed in detail in Neubecker, Heraldiek, 187-190, 194, 196.
See also Eygun, Sigillographie du Poitou, 132-148.; Pastoureau, Les Sceaux, 69.
 New, Seals, 27.
 Eygun, Sigillographie du Poitou , 92-112.; Pastoureau, Les Sceaux, 74-76.
 Harvey, “This Is a Seal,” 3.
 For more information on the maintenance, cleaning, and restoration of seals, see Steemers, Zegels, 68-69, 99-102.; New, Seals, 125-126.
 Ailes, “Seals and Sealing Practices,” 113; New, Seals, 111.
 For a detailed description of the process of the forgery, abuse of and fraud with seals, see Ewald, Siegelkunde, 225 -41.; Jenkinson, A Guide to Seals, 29-31.; New, Seals, 16.
 Seals, 31; Harvey and McGuinness, 1996, 22-26; New, 2010, 29-32 cited in McEwan, “The Challenge of the Visual.”
New, Seals, 31.
 The encountered problems, among others surrounding the choice of search terms (‘lily’, ‘fleur de lis’ or, more generally, ‘flower’?) and whether only the primary or also the secondary meaning of imagery should be included, are discussed in McEwan, “The Challenge of the Visual.”
 Jenkinson, A Guide to Seals, 56: “Speaking generally, in the case of private seals it is particularly difficult to give any precise guide to classes of the Public Records in which they may most profitably be sought: they may be expected, in fact, anywhere where a man may have had occasion to set his hand or seal to a document”.; New, Seals, 31.
Ailes, Adrian. “Seals and Sealing Practices.” Journal of the Society of Archivists 33, no. 1 (2012): 113-14.
Bedos-Rezak, Brigitte Miriam. “Medieval Identity: A Sign and a Concept.” The American Historical Review 105, no. 5 (2000): 1489-533.
Ewald, Wilhelm. Siegelkunde. München: Oldenbourg XIV, 1975.
Eygun, François. Sigillographie du Poitou jusque’en 1515: étude d’histoire provinciale sur les institutions, les arts et la civilisation d’après les sceaux. Mâcon: Protat Frères, 1938.
Harvey, P.D.A. “This Is a Seal.” Chap. 1 In Seals and Their Context in the Middle Ages, edited by Phillipp R. Schofield, 1-5. Haverton, PA: Oxbow Books, 2015.
Jenkinson, Hilary. A Guide to Seals in the Public Records Office. London: Her Majesty’s Stationary Office, 1968.
Laurent, René. Sigillografie. Brussel: Algemeen Rijksarchief, 1986.
McEwan, John Alexander. “The Challenge of the Visual: Making Medieval Seals Accessible in the Digital Age.” Journal of Documentation 71, no. 5 (2015): 999-1028.
Neubecker, Ottfried. Heraldiek: bronnen, symbolen en betekenis. Amsterdam: Elsevier, 1977.
New, Elizabeth A. Seals and Sealing Practices. Archives and the User. 11 vols. Vol. 11, London: British Records Association, 2010.
Pastoureau, Michel. Les Sceaux. Typologie Des Sources Du Moyen Âge Occidental 88 vols. Vol. 36, Turnhout, Belgium: Brepols, 1981.
Steemers, Ted. Zegels: geschiedenis, gebruik, conservering, restauratie. Maastricht: Rijksarchief in Limburg, 1984.
Tessier, Georges. Diplomatique Royale Française. Paris: Éditions A. et J. Picard et Cie., 1962.