The Boston Teachers Union says it has taken out a full-page ad in the Chicago Sun-Times in support of the Chicago teacher union in its contract battle with Mayor Rahm Emanuel.

Union president Richard Stutman says the union reacted after Emanuel “publicly chastised” Chicago’s union, comparing them with Boston teachers, who this week reached a contract settlement with the administration of Mayor Thomas M. Menino.

Stutman says Emanuel was wrong on the details and he “overstepped.”

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“We offer 100 percent support to our fellow unionists in Chicago,” he said this morning. “The issues they’re fighting for in Chicago are issues that are held dearly by millions of teachers around the country.”

“Thank you for mentioning our contract settlement, which came about as a result of a mutually respectful conversation between the parties” said the $6,000 ad, a copy of which was provided by the Boston union. “Perhaps you can learn from us—and begin to treat your own teaching force with the same respect.”

The Chicago mayor’s press office didn’t immediately have a comment.

After 27 months of contentious negotiations, the Boston union and the School Department reached a tentative agreement early Wednesday morning. It is expected to usher in more rigorous teacher performance evaluation and provide extra help for teachers who need it, the Globe reported Thursday.