Each detail sits side by side, and the one on the card you started from stays unless you press Use this under the other one. You can also type straight into any box to put in something new. For example, if Grandad was born in Leeds on one record and York on the other, keep Leeds, press Use this for York, or type the right town yourself.
Where to find it
Merge page → under the other record's box
Only people with the "Remove" tick, with Safe edit on.
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