Look at this link on "Fill Fields automatically on form based on a control's
value"

http://www.mvps.org/access/forms/frm0009.htm

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A better way is to create a combo that lists all the account, where the user
can select an account or type it.
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Set the Combo RowSource ro include both fields: account number and account's
title.
Select [account number], [account's title] From TableName
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Set the combo "Limit to list" property to yes so the user won't be able to
type the wrong account
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Set the text box ControlSource that display the title to
=[ComboName].Column(1)

Note: the combo column number start with 0
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