Think you know the answer to this one? Think again! What you know as an R-class ship may depend on which cruise line is your favorite.

Let's start with the simplest explanation first. Holland America Line has four ships in its Rotterdam class, all built between 1997 and 2000 specifically for the cruise line. These ships carry roughly 1,400 passengers and are frequently referred to as "R-class."

Where it gets interesting is that around the same time Holland America's ships took to the seas, Renaissance Cruises was building their own R-class fleet that sailed primarily in European waters, each about half the size of the Holland America ships. These ships, built between 1998 and 2001, were numerically named -- R One through R Eight. Renaissance Cruises ceased operations shortly after R Eight sailed.

The original Renaissance R-class ships now ply the world's oceans under different names but are sometimes still referred to by their original "R" designation. Oceania Cruises' Regatta class consists of the former R One, Two, Four and Five. Fans of Oceania frequently refer to these ships as "R Class," now named Insignia, Regatta, Sirena and Nautica, respectively.

Princess Cruises' Pacific Princess, while not typically referred to by its former R-class designation, was once R Three. The final three Renaissance ships make up the Azamara fleet -- Azamara Quest, Azamara Journey and Azamara Pursuit.

If you aren't confused enough yet, the original Renaissance Cruises also operated eight smaller yacht-style ships, with numerical names: Renaissance I through Renaissance VIII. Two of these ships now operate as expedition ships. Silversea's Silver Galapagos was once Renaissance III and Quark Expedition's Sea Spirit is the former Renaissance V.