### Pitch Histograms, Merged Pitch Histograms, and Their Anhemitonic Pentatonic Variants  
Pitch distributions have been used in computational music study since the 1960s and into the 1980s in various fields such as mathematical music structure analysis and computer-assisted generative composition [6; 7]. Subsequently, the concept of the pitch histogram, describing the pitch content of notes in music pieces, was systematically introduced in [8]. In [9], the authors summarize various variants of the pitch histogram and an extensive list of features extracted from them. MIR studies using pitch histograms emerge from time to time [10; 11; 12; 13].  
Each note has not only pitch information but also duration characteristics. Therefore, rhythmic value histograms [9], beat histograms [14; 15; 16; 17; 18], and (normalized) duration histograms [10] were created and play roles in MIR to describe the temporal information on a song's melody.  
Our study combines temporal information (pitch continuity) with pitch information. Therefore, in addition to applying the basic pitch histogram [8], we also utilized the recently proposed merged pitch histogram [19] to visualize songs' main melody statistics. As a variant of the basic pitch histogram, the merged pitch histogram joins successive identical pitches to be counted only once, one of the alternatives regarding how to \"count\" pitches, which has been debated and often noted in some computational musicology works but was systematically summarized and named for the first time in [19]. For our research context, firstly, the merged pitch histogram downplays the continuous repetition of pitches due to splitting words in songs of polysyllabic languages, making them more applicable to be analyzed together with songs of monosyllabic languages (e.g., Mandarin). It is helpful as we will conduct in Section 4 a classification study of traditional Chinese anhemitonic pentatonic folk songs together with songs of other languages around the world. Secondly, some interesting phenomena in Chinese folk songs, such as the characteristic onomatopoeia, use successive identical pitches, for example, the \"dong dong dong\" simulating a rattle-drum. Merging the successive identical pitches improves the statistical pitch analysis, which will be explained in Section 2.2.  
This study uses notation-based symbolic pitch histograms, and all histograms plotted are not normalized to facilitate the comparison between each pitch's merged and original bin heights. The non-normalized visualization has no impact on the histograms' shapes, the observation analysis, and the feature design in this study.  
Because of the characteristics of Chinese anhemitonic pentatonic music introduced above, we will generate movable do solflege-based basic and merged pitch histograms by replacing the pitch letters (A-G) or frequencies in the traditional pitch histograms with the movable do solflege (do-si), as shown on the horizontal axis ticks in Figure 1. In addition, we will also apply the anhemitonic pentatonic pitch histograms that retain only the pitches fitting the pentatonic scale, which also has its merged version, the merged anhemitonic pentatonic pitch histogram (see Section 2.2).