This topic raises a question about common usage.
In my world, an addendum, like an appendix, is a supplement to a book. Publications may conclude with several of these add-ons but not usually in combination. So what is acceptable usage when a book contains more than one addendum or appendix – Addenda? Appendices?
Conversationally, this sounds pretentious if not passé. It took a short twenty years for appendixes to become the preferred term. Updating the style guide neither hastens a term’s use nor demise.
I turn to Merriam-Webster for guidance. The dictionary recognizes the use of addendums but keeps silent on whether that form is correct. Answers.com says addendums is an acceptable plural.
These days when I hear appendices and especially indices the words ring like nails on a blackboard. (Does anyone even know what a blackboard is?) Addendums will become generally accepted no doubt. A living language gets tweaked every day.