Main document
When discussing a mail merge, a main document contains the primary text, images, and fields completed during the merging process. For example, with a letter advertising a new service, the main document contains the service information, but the customer information is stored externally from the main document. The names and other fields are inserted into the main document and printed when the merge happens. In this example, all customers get the same letter, but it feels more personal because it's addressed to them.
Main document example
Below is an example of a main document in Microsoft Word. In this example, each merge field was inserted into the document and is shown surrounded in angle brackets. When this letter is printed, each field is filled in from the associated recipient list.
<<First_Name>> <<Last_Name>>
<<Address>> Dear <<First_Name>>, We think our new service is perfect for you <<First_Name>> and think you should give it a try. Sincerely, Some company