In this paper, the study of systems that evolve in time by means of the comparison of time series is proposed. An improvement in the form to compare temporal series with the incorporation of qualitative knowledge by means of qualitative labels is carried out. Each label represents a rank of values that, from a qualitative perspective, may be considered similar. The selection of labels of a single character allows the application of algorithms of string comparison. Finally, an index of similarity of time series based on the similarity of the obtained strings is defined.