By default there is no column name for a column of row names. If col.names = NA and row.names = TRUE a blank column name is added, which is the convention used for CSV files to be read by spreadsheets. Note that such CSV files can be read in R by

write.csv and write.csv2 provide convenience wrappers for writing CSV files. They set sep and dec (see below), qmethod = "double", and col.names to NA if row.names = TRUE (the default) and to TRUE otherwise.

write.csv uses "." for the decimal point and a comma for the separator.

write.csv2 uses a comma for the decimal point and a semicolon for the separator, the Excel convention for CSV files in some Western European locales.

These wrappers are deliberately inflexible: they are designed to ensure that the correct conventions are used to write a valid file. Attempts to change append, col.names, sep, dec or qmethod are ignored, with a warning.

CSV files do not record an encoding, and this causes problems if they are not ASCII for many other applications. Windows Excel 2007/10 will open files (e.g., by the file association mechanism) correctly if they are ASCII or UTF-16 (use fileEncoding = "UTF-16LE") or perhaps in the current Windows codepage (e.g., "CP1252"), but the ‘Text Import Wizard’ (from the ‘Data’ tab) allows far more choice of encodings. Excel:mac 2004/8 can import only ‘Macintosh’ (which seems to mean Mac Roman), ‘Windows’ (perhaps Latin-1) and ‘PC-8’ files. OpenOffice 3.x asks for the character set when opening the file.

There is an IETF RFC4180 (https://tools.ietf.org/html/rfc4180) for CSV files, which mandates comma as the separator and CRLF line endings. write.csv writes compliant files on Windows: use eol = "\r\n" on other platforms.