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A company seal (sometimes referred to as the corporate seal or common seal) is an official seal used by a company. Company seals were predominantly used by companies in common law jurisdictions, although in modern times, most countries have done away with the use of seals.
Traditionally, the seal was of some legal significance because the affixing of the seal signified that the document was the act and deed of the company, whereas when a document was merely signed by a director, then that was deemed to be an act carried out on behalf of the company by its agents, which was subject to applicable restrictions and limitations under the ordinary law of agency.
Corporate seals are generally only used for two purposes by corporations today:
Physically, seals used to be used to make an impression on melted wax on the relevant document, although modern seals will usually only leave an indentation or impression on the paper (although sometimes a red wafer is used to imitate old red wax seals, and to make the sealing show up better on photocopies).