I wanted to continue along a certain line of research.

I've built my whole world to work along certain lines.

But you cross a certain line, there's no going back.

Other private companies own exclusive rights to run services on certain lines.

"Do you have to be in a certain line of work to welcome Earth men!"

Some things are more personal than others, he said, and "I don't want to cross a certain line."

It had to go along certain lines once we'd set out.

Before, there was the feeling that you have to vote a certain line.

Indeed, within certain lines, he had the right to keep an eye upon her as his charge.

You can see them jumping out of the way on certain lines.