Naturally, it falls to the discretion of the mistress of an establishment, to decide whether her servants should knock only at doors in specific circumstances or more widely. However, in Society, there are some generally accepted rules which most households follow, and we shall endeavour to lay them out below.
To commence, it is roundly held to be most vulgar and wrong for a servant to knock at the door of any room which might be considered, in a manner of speaking, public. Thus, the rooms at which a servant does not knock upon the door include the library, smoking room, drawing room and dining room. The very point of knocking is to save both the person inside the room, and the one outside, any embarrassment or discomfort. In any respectable abode, nothing of an embarrassing nature could possibly transpire in a public room. And, consequently, there is no need to knock.
Yet, with regard to the drawing room, special mention should be made of the etiquette of announcing callers. The servant should not knock at the door, but should open it directly and step well into the room. Ideally facing their mistress, they would then announce the caller. The servant should not hide themselves away behind the door.
The rooms at which a servant does knock are the bedroom and dressing room. Even if a servant has been rung for, they should still knock to announce their arrival and then await further instruction. For example, if refreshment has been sent for, they might be directed to bring it into the room. However, they could also be told to leave it outside.
The manner with which a servant knocks also requires our attention. No one wants their servant to hammer on doors in a coarse and inelegant fashion. Instead, a servant should knock smartly and clearly upon doors with their knuckles.
Anon., Manners and Rules of Good Society; Or, Solecisms to be Avoided, [Leopold Classic Library].
Howard, Lady Constance, Etiquette: What to Do and How to Do It, (London: F. V. White & Co., 1885), [ULAN Press].
Motherly, Mrs., The Servant’s Behaviour Book, (London: Bell and Daldy, 1859).
 Lady Constance Howard, Etiquette: What to Do and How to Do It, (London: F. V. White & Co., 1885), [ULAN Press], p. 217.
 Mrs. Motherly, The Servant’s Behaviour Book, (London: Bell and Daldy, 1859), p. 79.
 Anon., Manners and Rules of Good Society; Or, Solecisms to be Avoided, [Leopold Classic Library], p. 40-1.
 Ibid., p. 41.
 Howard, Etiquette: What to Do and How to Do It, p. 217.
 Mrs. Motherly, The Servant’s Behaviour Book, p. 79.
 Ibid., p.80.