# Add custom information to text citations with biblatex

I've come across a case now where I need to refer to items in my bibliography as well as to something someone has told me personally. I would like that text citation to look something like this:

I know from books and people I have talked to that George Harrison played the guitar in the Beatles (Lennon 1972, pp. 48 sqq.; Starkey 1994, pp. 103 sqq; Paul McCartney, p.c.).

The "Paul McCartney, p.c." part needs to be custom added when needed. How should I do this with biblatex? Below a MWE.

\documentclass{article}
\usepackage[style=authoryear]{biblatex}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@book{lennon1972,
AUTHOR = "John Lennon",
TITLE = "Who did what in the Beatles",
YEAR = "1972"}
@book{starkey1994,
AUTHOR = "Richard Starkey",
TITLE = "I'm the drummer",
YEAR = "1994"}
\end{filecontents}

\begin{document}
I know from books and people I have talked to that George Harrison played the guitar in the Beatles \parencites[48\psqq]{lennon1972}[103\psqq]{starkey1994}[Paul McCartney p.c.]{}

\printbibliography
\end{document}


The plural form of the various citation commands, namely \cites, \Cites, \parecites, \Parecites, \footcites, \footcitestext, \smartcites, \Smartcites, \textcites, \Textcites, \supercites, \autocites, and \Autocites all take the same basic form (see §. 3.7.3 in the manual):

\cites(<pre>)(<post>)[<pre>][<post>]{<key>}...[<pre>][<post>]{<key>}


The (<pre>)(<post>) are sort of 'global' arguments that precede or follow the entire citation list. Thus one could use:

\parencites(Paul McCartney p.c.)[48\psqq]{lennon1972}[103\psqq]{starkey1994}


which is equivalent to:

\parencites()(Paul McCartney p.c.)[48\psqq]{lennon1972}[103\psqq]{starkey1994}


If one wants only a global pre-note, then you need to supply an empty post-note

\parencites(Paul McCartney p.c.;)()[48\psqq]{lennon1972}[103\psqq]{starkey1994}