>>I select a child record (order detail line 1) in a grid on the parent form. Then I click edit. The child edit screen opens. I change a field called partnum from "11X" to "12X". I click "save" on child form. I have now returned to parent form. Then I realize I want to change order detail line 1 again. So I edit the same record. Now I change partnum to "13X". Then I decide not to change the record and I want partnum left as "12X" so I click "cancel" on the child form. So what value do you expect partnum to be and how does it get the value? I cannot tablerevert because that will change partnum back to "11X".
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>What does you Save button do on the Child form? Does it tableupdate the child or simply closes the child form? If the former, than your Cancel button would work just fine in your scenario.

The save button on the child would just close the form (and run the record validation). I do not want to tableupdate the child. That is handled when save is clicked on the parent form. If cancel is clicked on the parent I expect the changes on both the parent and child to be reverted (which is easy to do with tablerevert).