At, In and On cause difficulty when used in phrases giving the place or locality of an action.
When reference to the interior of any place is made prominent, in is used; when a place is regarded as a mere local point, at is more commonly employed; when the direction is indicated, on is sometimes used in place of at.
- look for a book in the library
- meet a friend at the library
- sit on my right
- the town lies on the east coast
- he appointed regular meetings of the States of England twice a year in London
—Hume - an English king was crowned at Paris
—Macaulay
In is commonly employed before the names of countries or districts and at before names of institutions, public offices, or business houses.
- in America
- in the South
- Milton was educated at Christ’s College
- at the customhouse
- at the jeweler’s
With names of towns and cities the choice between in or at usually depends upon whether the place designated is felt respectively
(1) as an including area or scene, especially with an implication of destination or permanence of occupancy, or of having familiar associations for the speaker or
(2) merely as a point (as along a journey or course) on a map or in space or at a remove from the speaker.
- on our way to visit in Troy we lunched at Albany
- after a stopover at Chicago, we arrived in Sioux Falls on Friday
- a man born here in Zenith is consul at Hong Kong
In giving a town address we say at 141 Wood Street in Springfield. In giving the street without the number, in is preferred in Great Britain, on in the United States.
At, In and On are clearly distinguishable when used to introduce a phrase giving the time of an action. When reference is to time by the clock or to a point of time registered by a clock, at is commonly used.
- at two o’clock
- promptly at the hour appointed
- at three minutes to six
When the reference is not to a point but to a period in the course of which an action occurs, in is the usual preposition.
- at two o’clock in the afternoon
- September 1 st in the year 1939
- in the month of May
When the reference is to a particular day in the course of which something occurs, on is used.
- on July fourth there will be a celebration
- it happened on a Sunday
On is sometimes used also with reference to a point of time with which there is, or should be, coincidence.
- be here on the hour
- he is always on the dot