Two parallel lines with varying slopes and constant distance ( say 4 units) between them.

Let D be a line with equation $y=ax+b$, with a a varying number.

So D has a changing slope and is rotating about point $P=(0,b)$ .

Let D be a line with equation $y=ax+b$, with a a varying number.

So D has a changing slope and is rotating about point $P=(0,b)$ .