Much of linguistic analysis rests on a single key question: given entities X and Y as objects for analysis, are they the same or different? This issue pervades all components of grammar: phonetics, phonology, morphology, syntax, etc. Moreover, in addressing this issue, one often needs to recognize the relevance of different levels of analysis, especially underlying versus surface, since underlying sameness can be surface difference, and vice versa. For example, phonemic analysis takes phones

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... h decidedly different phonetic realizations (e.g. aspirated vs. unaspirated stops in English) and treats them as the same at the phonemic level if their distribution does not overlap. But at the same time, segments that seem to be the same phonetically on the surface and even phonemically as well, e.g. the [d] of recede and the [d] of invade, might need to be treated as different from a morphophonemic standpoint, since, in this example, the former alternates with [s] in the related noun recession whereas the latter alternates with [z] in invasion, both nominal formations having ostensibly the same suffix. In syntax, too, patterns that are alike on the surface, such as control constructions and raising constructions, can show some unlike properties that lead, in most current theoretical frameworks at least, to structural differentiation in some way, e.g., in underlying structure. An answer to the above key question regarding sameness often involves a recognition of differences too. Thus, the issue becomes one of measuring similarities and differences against one another and weighing the relative importance of one or the other, as well as deciding how to represent the sameness or difference that one ends up positing.