My CSV opens with everything in one column

The file opened, but every row sits in a single column with the separators still visible in the text. Widening the column shows the whole line, commas or semicolons and all.

Why this happens

• The name of the format promises a comma and the file often does not contain one. Exports written in locales where the comma is already the decimal mark use a semicolon instead, so that 1,50 stays one value.
• Which separator a spreadsheet expects when you double-click a file comes from regional settings rather than from the file. This is why the same CSV opens correctly on a colleague's machine and in one column on yours, with neither of you having done anything differently.
• Some exporters try to settle it by writing a sep= line as the first line of the file. It works in the tools that look for it, and in the tools that do not it becomes a spurious first row.
• A tab-separated file saved with a .csv extension produces the same symptom for the same reason. The extension is not a description of the contents.

What to do right now

• Open it through the import dialog rather than by double-clicking: Data → From Text/CSV, where the delimiter is something you choose rather than something you inherit.
• Look at the raw file in a text editor to see which separator it actually uses before choosing. It takes a few seconds and removes the guesswork.
• Do not fix it by replacing semicolons with commas across the file. In a file that uses semicolons because commas are decimal marks, that turns every price into two columns.

Stopping it on the next file

The delimiter is sniffed per file rather than assumed, and a tab wins over a comma, which wins over a semicolon, so a European semicolon export and a tab-separated paste land the same way a plain CSV does. A byte order mark at the start is stripped rather than being treated as part of the first column heading. That happens on each file independently, which matters when two suppliers export from tools with different conventions into the same sheet.

The vendor price list template is a sheet with those rules already set, if you want to see the shape of one before building your own.