Gentleman Usher and Lady Usher are titles for some officers of the Royal Household of the United Kingdom. For a list of office-holders from the Restoration of the monarchy in 1660 up to the present day see List of Lady and Gentleman Ushers.
Gentleman Ushers were originally a class of servants found not only in the Royal Household, but in lesser establishments as well. They were regularly found in the households of Tudor noblemen, and were prescribed by Richard Brathwait, in his Household of an Earle, as one of the "officers and Servants the state of an Earle requireth to have". The Gentleman Ushers occupied an intermediate level between the steward, the usual head, and the ordinary servants; they were responsible for overseeing the work of the servants "above stairs", particularly those who cooked and waited upon the nobleman at meals, and saw to it the great chamber was kept clean by the lesser servants. He was also responsible for overseeing other miscellaneous service, such as the care of the nobleman's chapel and bed-chambers. It was traditionally the Gentleman Usher who swore in new members of the nobleman's service. 
The duties of a Gentleman Usher, not unlike those of a contemporary butler, made him quite important in Tudor and 17th-century households. George Chapman's play The Gentleman Usher has as its title character the pompous but easily fooled Bassiolo, Gentleman Usher to Lord Lasso.
The Gentleman Ushers of the Royal Household, in order of precedence, were originally the four Gentleman Ushers of the Privy Chamber (who attended the Sovereign in the Privy Chamber), the four Gentleman Ushers Daily Waiters, and the eight Gentleman Ushers Quarter(ly) Waiters. The latter two originally served different terms of service, but the distinction later became only nominal, as the role of the Gentleman Ushers became increasingly ceremonial and they exercised less supervision over the staff. In 1901, King Edward VII abolished the three classes and began to appoint simply Gentleman Ushers in Ordinary. The first Lady Usher of the Black Rod was appointed in 2017. The first Lady Usher in Ordinary was appointed in 2021.
Today an establishment of 10 Lady and Gentleman Ushers is maintained for attendance at royal events.  Lady and Gentleman Ushers to The King are generally appointed from retired military officers with three representing the Royal Navy, four representing the Army and three representing the Royal Air Force.
When on duty Ushers generally wear either service uniform with a brassard displaying the royal cypher or morning or evening dress, depending on the occasion. They receive a modest honorarium for the upkeep of their orders of dress.
Among their duties, they act as ushers at Royal Garden Parties and Investitures as well as on State occasions. At royal weddings, funerals, coronations and other large church services they may be called upon to lead royal and other important guests in procession before conducting them to their seats. Occasionally they may be called upon to attend an event (e.g. a memorial service) as the monarch's representative.
Ushers retire at 70, when they may become Extra Gentleman Ushers.
Certain Gentleman Ushers have duties outside of the Royal Household, usually attached either as officers of an order of knighthood or to a House of Parliament. These are, in order of antiquity:
Gentleman Ushers of the Black Rod also exist for New Zealand, Australia and its states, and Canada. In some respects, the Military Social Aides to the US President, who attend on some 2 to 4 afternoons a month to assist visitors to the White House, are an American and more recent equivalent to the Gentleman Ushers in Ordinary.
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