Fence Boundary

Fence

A fence is a type of barrier. It's an upright structure that is often made of wood or metal and separates two areas.
The ideal English house was a thatched cottage with a large garden enclosed by a fence. A dog could run freely in the garden because the fence would keep him in.
Tim and Mr. Wilson used to chat every night through the fence, but Tim never saw his neighbor's face because the fence was too high.
When someone cannot choose a side in a dispute or hasn't formed an opinion on a controversy, we say they are on the fence.

Boundary

A boundary is a line or limit dividing two spaces, for example two different pieces of property. A boundary may also describe an unofficial or unwritten rule about what is acceptable behavior. Telling someone this kind of rule is called setting a boundary or setting boundaries.
Our new neighbour, Mr. Jackson, pointed to the large oak trees across the garden and explained that they marked the boundary between our properties. Beyond the oak trees is where his property begins.
My grandmother explained that it's very important to set boundaries for children at home. If they don't learn early on what behaviour is acceptable and what is not, they can be very difficult and aggressive at school.
In sports, when a ball or player goes beyond the official boundary we say they are out of bounds. When someone does or says something mildly offensive or against the norm, we may also say they are out of bounds.