appears in large type above the staff, it functions as a new tempo, and thus implies an immediate change.) Several terms, e.g., assai, molto, poco, subito, control how large and how gradual a change should be (see common qualifiers). After a tempo change, a composer may return to a previous tempo in two ways: a tempo – returns to the base tempo after an adjustment (e.g. ritardando ... a tempo undoes the effect of the ritardando). Tempo primo or Tempo Io – denotes an immediate return to the piece's original base tempo after a section in a different tempo (e.g. Allegro ... Lento ... Moderato ... Tempo Io indicates a return to the Allegro). This indication often functions as a structural marker in pieces in binary form. These terms also indicate an immediate, not a gradual, tempo change. Although they are Italian, composers tend to employ them even if they have written their initial tempo marking in another language. Tempo–rhythm interaction One difficulty in defining tempo is the dependence of its perception on rhythm, and, conversely, the dependence of rhythm perception on tempo. Furthermore, the tempo-rhythm interaction is context dependent, as explained by Andranik Tangian using an example of the leading rhythm of ″Promenade″ from Modest Mussorgsky's Pictures at an Exhibition: This rhythm is perceived as it is rather than as the first three events repeated at a double tempo (denoted as R012 = repeat from 0, one time, twice faster): However, the motive with this rhythm in the Mussorgsky's piece is rather perceived as a repeat This context-dependent perception of tempo and rhythm is explained by the principle of correlative perception, according to which data are perceived in the simplest way. From the viewpoint of Kolmogorov's complexity theory, this means such a representation of the data that minimizes the amount of memory. The example considered suggests two alternative representations of the same rhythm: as it is, and as the rhythm-tempo interaction — a two-level representation in terms of a generative rhythmic pattern and a “tempo curve”. Table 1 displays these possibilities both with and without pitch, assuming that one duration requires one byte of information, one byte is needed for the pitch of one tone, and invoking the repeat algorithm with its parameters R012 takes four bytes. As shown in the bottom row of the table, the rhythm without pitch requires fewer bytes if it is “perceived” as it