If you insert a right-aligned tabstop after your content control, positioned where you want the underline to end, then insert a tab after the content control and format both the content control and the tab as underlined, the underlining will remain the same whether the form is filled out electronically of by hand. You can achieve a similar effect by putting the content control in a table cell with a bottom border.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]