The RIR breed is acceptable in either the single comb or the rose comb variety. The single comb was admitted to the SOP in 1904, one year earlier than the rose comb. Below is information from the 1998 SOP regarding the rose comb:
"The Rose Comb Rhode Island Red was admitted to the Standard in 1905, although some breeders acclaim it to be the original variety. The earlier Rhode Island Reds sported both the single and rose combs, some even having pea combs, due to their mixed ancestry and the fact of their being bred primarily for market purposes before they became a Standard breed." In the Shape description for the male, it states about the rose comb: "Rose; moderately large, firm on head; oval, free from hollow center, surface covered with small rounded points, terminating in a spike at the rear, the spike drooping slightly but not conforming too closely to the shape of head."
There are many parts of the country where the RC RIR is almost never seen, and with the popularity of the SC RIR, it is understandable why many people would assume that the SC is the only variety of RIR in the Standard. There is also a Rhode Island White, admitted in 1922, with the Standard being exactly the same as for the Red, except of course for color. It appears, although is somewhat open to interpretation based on how the write-up is organized, that the RIW is only acceptable with a rose comb.