at the frontier , also: at a frontier
on the frontier , also: on a frontier

On the frontier a man and his wife are two.

He did come from a family with deep history on the frontier.

And do you go to the army on the frontier.

They found themselves on the frontier, in more pain than ever.

Do you have what it takes to live on the American frontier in 1883?

I don't think we see evidence of it yet out on the frontier.

Thomas was left to make his own way on the frontier.

During the War of 1812, he served on the western frontier.

His life was on the frontier with the black people he loved.

Government, the group states, has an important role to play on the electronic frontier.