The Big Four includes the four largest international professional services networks in accountancy and professional services. These professional services networks handle the wide majority of audits for publicly traded companies and various private companies thus creating an oligopoly in large companies’ auditing. This ‘Big Four’ group was once known as the ‘Big Eight’, and got reduced to the ‘Big Five’ through a series of mergers. The ‘Big Five’ further turned to ‘Big Four’ through further mergers.
None of the Big Four accounting firms is an individual firm. They are all accounting networks, each one being a network of firms, held and managed independently and have entered into agreements with other firms existing in the network to share a common name, brand, and quality standards. Each network has set up an entity aimed at co-ordination of activities of the network. In one case (KPMG), Swiss is the co-coordinating entity, while in the remaining three (Deloitte Touche Tohmatsu, PwC and Ernst & Young), UK is the coordinating entity.