draw a line

draw a line or draw the line  {v. phr.}

1. To think of as different.

The law in this country draws a line between murder and manslaughter.

2. To set a limit to what will be done; say something cannot be done.

We would like to invite everybody to our party, but we have to draw a line somewhere.

- Often used with "at".

Mrs. Jones draws the line at permitting the children to play in their father's den.