to be on a first-name basis
Sich mit der vertraulichen Anrede 'du' ansprechen.
To use the informal 'du' instead of the formal 'Sie' with each other.
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In unserem Team sind alle per du.— In our team, everyone is on first-name terms.
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Wir kennen uns lange und sollten endlich per du sein.— We've known each other a long time and should finally be on first-name terms.