^Lichine, Alexis (1987). Alexis Lichine's New Encyclopedia of Wines & Spirits (5th ed.). New York: Alfred A. Knopf. p. 236. ISBN0-394-56262-3.
^DuBose, Fred; Spingarn, Evan (2004). The Ultimate Wine Lover's Guide 2005. Barnes & Noble. p. 202. ISBN9780760758328. Retrieved 6 June 2020. Exactly when stronger wines or spirits began to be added to wine to preserve it is lost to history, but it worked — and fortified wine was born. History does record how the fortified wines Port and Madeira came to be.