Prepositions and places: IN, AT, ON

The U.S. president lives in The White House at number 1600 on Pennsylvania Avenue in the city of Washington in the District of Columbia in the United States on Earth. If these are all places, why does English need three different prepositions (in, at and on) to introduce them? Fortunately, the answer follows from general rules about when each of these prepositions should be used.


IN (AREA)
Use in when the place is thought of as a containing area or time period.

Space:

He lives in this house/cave/hole.
He lives in this city/state/province/country/region/galaxy.
The point is in this range/area/space. [a 1, 2 or 3 dimensional container]

Time:

He lived in the seventeenth century.
He is in his thirties.

AT (POINT)
Use at when the place is thought of as a point.

Space:

The US president lives at number 1600 Pennsylvania Avenue.
There is a black hole at the center of our galaxy.
It stopped at the end/edge/membrane. [a point where a boundary could be crossed regardless of whether the boundary itself is a point, line or surface]

Time:

He lived at the end of the seventeenth century.
The show starts at eight o'clock.

What is considered a point depends on the scale at which it is viewed. Boundaries in time like the beginning and end of events always have no duration, but a period of a few days could also be seen as a point when viewed on a larger time scale. Hence, we say things like at Christmas time, which is understood as a time of year, and relatively brief on that scale.

ON (SURFACE)
Use on to indicate contact with a surface. This is usually a two dimensional surface, but on can also be used to indicate contact with a line that is viewed as a boundary, and very occasionally to indicate contact with a point.

Space:

We live on Earth. [contact with a surface]
He lives on the third floor. [contact with a surface]
There are goats living on the mountain. [contact with a surface]
There are spiders living on the ceiling. [contact with a surface]
The cup is on the table. [contact with a surface]
He is on the edge/border. [contact with a line]
He stood on this spot. [contact with a point]

Time:

We had lunch on Monday. [imagine the days of the week as stepping stones]

Sometimes, you can use more than one preposition depending on whether you want to speak about areas, points or surfaces:

He lives in Pennsylvania Avenue. [understood as a containing area]
He lives on Pennsylvania Avenue. [understood as a contact with a surface]