Special Realizations of Morphemes: Unique Morphs or cranberry morphs
Although we have clearly distinguished between two types of morphemes in the previous sections, we will now introduce a third type of morpheme: the unique morph.
Unique morphs violate the clear-cut definition of morphemes in free and bound morphemes. In many ways, they resemble bound morphemes because they cannot meaningfully stand alone but,still, contribute meaning to a word. In laughter, the unique morph -ter might be thought of as being a bound suffix because it works similar to suffixes like -age and -ion. The morpheme cran in cranberry does not carry meaning when in isolation, yet it defines the kind of berry when attached to the root berry. Because this is a popular example of unique morphs, a second term for this type of morpheme is cranberry morph. In addition, cranberry shows another peculiarity of unique morphs. Although cran is not a free morpheme, it is considered as a root because -of its similarity to the roots in black-berry, blueberry, snow-berry or wax-berry. Cran is decisive because it differentiates the (cran)berry from any others.
(examples taken from Bauer 1988, 40)