The three parts of every merge
- The data source: a list, spreadsheet or database with one row per recipient and one column per detail. Microsoft names Excel sheets and Outlook contacts as the usual sources; Gmail reads a Google Sheets file.
- The main document: the letter, email or label sheet, with placeholders where personal details go. Word calls them merge fields; Gmail calls them merge tags and shows them as @firstname and similar.
- The merge: the step that produces one finished copy per row, either as printable pages, envelopes and labels or as separate emails sent straight from the program.
The column headings of the source become the names of the placeholders, so a heading like First name or a sheet with two header rows causes more trouble than any setting in the merge wizard. In Gmail, Google's help page says merge tags come from those headings and that the contact details must sit on the spreadsheet's first tab as plain text. Word's email merge needs a MAPI-compatible mail program installed, and each message goes out carrying just one recipient. Gmail makes the same promise differently: each person receives a separate copy and replies arrive in separate threads.
Check the output before it leaves
Treat the merge like any other output built from a list. Clean and check the source first, the way a pivot table would demand, preview a handful of results before sending the lot, and keep the merged document so there is a record of what went out. Where the same letter goes out every week on a trigger, a cloud flow is the sturdier tool, and where the mailing is really marketing, marketing automation handles consent and opt-outs that a merge ignores. The documents and word processing topic lists the merge alongside styles, sections and references.