Reference for Eruvin 51:3

איזו היא חדשה ואיזו היא ישנה חדשה שהוקפה ולבסוף ישבה ישנה ישבה ולבסוף הוקפה והאי נמי כהוקפה ולבסוף ישבה דמי

What is meant by a 'new [town]' and what by an 'old one'? A new [town is one] that was first surrounded [by a wall] and subsequently settled, and an old [town is one that was first] settled and subsequently surrounded [by a wall].

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