IF you have the mergefields aligned with tabs AND there is sufficient space
between the tabs for the data that will be merged, a mergefield in a record
that contains no data will NOT affect the layout.
However, instead of tabs, insert the mergefields inside the cells of a table
with fixed column dimensions. Then if the data in one field is too long for
the cell, it will word-wrap within the cell.

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Hope this helps,
Doug Robbins - Word MVP
> When I do a mail merge (Word letter/Excel data), my finished letters
> contain
> unpredictable output. The data does not align on tabs. Some data are
> empty
> but I'd expect the next field to be on the following tabs. How to make a
> word tab behave?