red line

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red line (plural red lines)

1. (ice hockey) The thick red line on the ice which divides the rink in half.
2. (figuratively) A boundary or limit which should not be crossed.
   • 2013 May 4, Daniel Byman, “Mr. Obama, Don’t Draw That Line”, in The New York Times‎, →ISSN:

     The administration’s ultimatum now seems like cheap talk, and it illustrates the risks of carelessly drawing red lines and issuing highly public threats that won’t be enforced.

   • 2023 September 6, Christian Wolmar, “Rail strikes: little prospect of negotiations”, in RAIL, number 991, page 44:

     After a failed attempt by ministers to bounce the union into an agreement by announcing its terms (which included all sorts of union red lines such as drivers having to pay for their own training) in the media rather than over the negotiation table, there is an understandable reluctance on the part of the unions to engage in any discussions unless there are no preconditions.

• blue line
• goal line (“thin red lines in front of the goals”)
• green line
• redline (verb)
• single yellow line

• “red line”, in Collins English Dictionary.
• “red line”, in Merriam-Webster Online Dictionary, Springfield, Mass.: Merriam-Webster, 1996–present.
• “red line”, in Cambridge English Dictionary, Cambridge, Cambridgeshire: Cambridge University Press, 1999–present.

• Nile red, relined