This paper focuses on different types of word­-formation uniqueness. Word-formation uniqueness can be found in solitary word­-formation structures, which the author defines as unique structures representing a specific word­-formation type found in a single derivative. The basic and simplest type is represented by a unique affix in a single derivative in the respective language of a given time period. An example of this is the Czech noun obličej ‘face’: the circumfix ob-ej (with -lič- as a radix to be found in the noun líce ‘cheek’) is found in no other noun in the contemporary Czech. Another uniquely formed noun in contemporary Czech is the noun rukáv ‘sleeve’, derived from ruka ‘hand’ with the unique suffix -áv. Specific word­-formation solitaires are theoretically conceived values. They are derivatives with a specific, but not uniquely found affix which is uniquely added to the stem of a single word of a specific part of speech, e.g. the very specific suffix -ule is added to the stem of the verb vrtět ‘to wag’ so that the noun vrtule ‘propeller’ is derived. This article shows that the notion of word-­formation solitariness can be theoretically postulated on different levels of abstractness, both of the form and of the word-­formation meaning.