Additive rhythm features nonidentical or irregular durational groups following one another at two levels, within the bar and between bars or groups of bars. This type of rhythm is also referred to in musicological literature by the Turkish word "aksak", which means "limping". In the special case of time signatures in which the upper numeral is not divisible by two or three without a fraction, the result may alternatively be called "irregular", "imperfect", or "uneven" meter, and the groupings into twos and threes are sometimes called "long beats" and "short beats".