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

The gate at the frontier had been pulled over the road and locked shut.

Although only nine million pounds were sold in 1997, they are at the frontier of the mushroom industry today.

And we might have an American reception committee waiting for us at the frontier.

Now another group appeared at the far frontier of the star system, still an hour distant.

However, a careful watch at the frontier denied these the opportunity.

But he himself continues to stop at the frontier.

At the frontier, the declaration should be made when the last of the containers has arrived.

This enables checks to be carried out at the frontier.

We don't want them to turn us back at the frontier.

"Everything is always in trouble at the frontier," he said.