[draw] (the|a) line

to define a limit concerning what action or thing one refuses to do or tolerate

The idiom "draw the line" originated from the act of physically drawing a line to demarcate a boundary or limit, symbolizing a clear separation between two sides. Over time, the phrase evolved into a metaphorical expression, often used to signify setting a limit, establishing a point beyond which one is unwilling to go, or indicating a boundary that should not be crossed. It is commonly employed to convey a firm decision or establish a clear distinction.

After numerous disagreements, they decided to draw a line and stop discussing politics to maintain their friendship.
The manager had to draw the line when it came to excessive tardiness, implementing a strict policy to ensure punctuality.
It's essential to draw a line between constructive criticism and personal attacks during a debate.
The teacher had to draw a line regarding acceptable behavior in the classroom to maintain a conducive learning environment.
In negotiations, it's crucial to draw a line on certain terms to protect your interests and reach a fair agreement.

to make a clear distinction between two things, particularly two ideas, that are closely related

The idiom "draw the line" originates from the act of physically drawing a line to create a clear separation between two objects or areas. Figuratively, this idiom is used to emphasize the need to create a clear separation or demarcation, often to avoid confusion or maintain clarity in a given context.

In a healthy relationship, it's important to draw the line between constructive criticism and personal attacks.
The teacher had to draw the line when the students' playful banter started disrupting the class.
As a manager, I need to draw a line between being friendly with my employees and maintaining professional boundaries.
While it's crucial to encourage creativity, we must also draw a line to ensure adherence to ethical guidelines in the field of scientific research.