Tradition can also refer to beliefs or customs that are Prehistoric, with lost or arcane origins, existing from "time immemorial". Originally, traditions were passed orally, without the need for a writing system. Tools to aid this process include poetic devices such as rhyme and alliteration. The stories thus preserved are also referred to as tradition, or as part of an oral tradition. Even such traditions, however, are presumed to have originated (been "invented" by humans) at some point. Traditions are often presumed to be ancient, unalterable, and deeply important, though they may sometimes be much less "natural" than is presumed. It is presumed that at least two transmissions over three generations are required for a practice, belief or object to be seen as traditional. Some traditions were deliberately invented for one reason or another, often to highlight or enhance the importance of a certain institution. Traditions may also be adapted to suit the needs of the day, and the changes can become accepted as a part of the ancient tradition. Tradition changes slowly, with changes from one generation to the next being seen as significant. Thus, those carrying out the traditions will not be consciously aware of the change, and even if a tradition undergoes major changes over many generations, it will be seen as unchanged.