There are two different scale systems used in Balinese gamelan: slendro and pelog. It is important to note that these are not scales with specific pitches, or even categories of scales (such as major or minor) that have specific interval relationships. There is no specific norm for either type of scale; instead, each is a system of guidelines for intervals within an octave; instrument-makers are free to interpret the guidelines as they wish, as long as the scale is consistent within a gamelan.

The slendro system uses five notes within each octave that are of roughly equal distance from each other. A very rough approximation of a slendro scale might be the Western notes A C D E and G (a pentatonic scale fairly familiar to Westerners), with the C, D and G tuned noticeably lower than equal temperament, so that the major second intervals and minor third intervals are more (but probably not exactly) equal.

The pelog system uses seven notes within an octave, with unequal intervals between them. The tuning tends to be close to that of the Western phrygian mode (E to E on the white keys of a piano). Some types of gamelan have all seven pelog notes available, but these usually do not use all seven pitches in one composition. Most commonly, groups of pitches are used as modes (for example, only pitches 1, 2, 3, 5 and 6, or only 1, 2, 4, 5 and 6 of that approximate-phrygian scale). Some gamelan only have one pelog mode available.

For rehearsal purposes, the notes of a scale may simply be numbered 1-5 or 1-7. When discussing theory, the five notes of a slendro or pelog mode are more likely to be named as dong, deng, dung, dang, and ding.

The tuning tradition in Indonesia is so different from that of Western tuning, that a gamelan may sound at first to a Westerner as if it is "out of tune", or as if the tuning is very sloppy, but neither is true.

As mentioned above, it is not considered necessary for every gamelan to have the same tuning. In fact, the variation in tuning of different gamelan using the "same" scale type is quite large; and the specific tuning of one gamelan is considered to be an important part of its unique sound, its musical personality.

The fact that the intervals are tuned differently from each other, and differently from Western equal temperament scales, probably leads to some of the initial Western discomfort with gamelan tuning, but an even more basic difference lies in differing concepts of what "in tune" means. Common practice in Western music not only insists on uniformity in tuning between groups, but also on extreme tuning precision within a group. If a few violinists play together, and they are not playing pitches that are almost exactly the same frequency, the result is considered to be painfully out of tune. This insistence on precise pitches at very consistent intervals allows complex harmony and counterpoint to sound good, but it is an insistence that is not shared by all musical traditions. Many musics that do not feature complex harmony and counterpoint are traditionally played with "wide tuning". In wide tuning, a broader band of frequencies is perceived by musicians and listeners to be a single "in tune" note, and in fact a note with too narrow a range of frequencies is heard, in these traditions, as being thin and lifeless. In other words, the width of the tuning is an important part of the timbre of the sound.

In many such traditions, the particular pitches that are played within the wide tuning are not important. Gamelan tuning is a particularly fascinating case of this preference, because its ideal is actually an extremely precise version of wide tuning. In order to produce a wide-tuning sound that is considered full and lively, the instruments in a gamelan often come in pairs. Each pair of instruments is very carefully tuned so that when they play together, the very slight difference in their pitches will produce audible beats in the sound. Like a piano tuner, who uses the rate of beats to tune the strings of a piano, the gamelan maker/tuner also listens to the speed of the beats. (The piano tuner listens for beats between different notes, rather than within the same note.) In an ideal ensemble, the beat rate is the same over the entire range of all of the instruments, a most impressive feat of tuning. The beat rate of the ensemble (typically between 5 and 8 beats per second) then becomes another part of its unique characteristic sound; it is so important that a tuner may choose to alter the pitch from one octave to the next rather than altering the beat rate. (Such choices may be forced, because the difference in tuning required to produce a specific beat rate changes as the frequency changes.) Listen closely to the long notes in this example to hear the beats produced when a pair of gender instruments play the same part together.

Due to the modal scales and the variations in tuning, Western-style functional harmony is not possible. However, gamelan music often sounds as if it includes harmony rather than just a single melodic line. This is due to the thick texture built of complex layers of melody, often with the more embellished parts playing different pitches at the same time. (See Form and Texture for more about this.)

For any traditional gamelan piece, there is usually a basic version of the melody, which is very closely bound to the form of the piece. There are also many variations on the melody, including versions that include a halt in the rhythm, to give a feeling of cadence, as well as a variety of more or less embellished versions, which often have two, four, or even eight times as many notes as basic versions of the melody. The signature texture that makes gamelan music so unusual and easily recognizable is created by having different instruments play different versions of the melody at the same time.

If everything that happens in a piece of music is new, the result sounds too random; some sort of repetition is a very important aspect of all kinds of music. Although repetition is an important aspect of Western music, most Western forms are linear in concept. As a piece develops, a previous section may return, as a whole or only in part, with or without substantial changes, and possibly with new, unrelated material introduced between repeated sections, or even interrupting sections. (See the course Sound Reasoning for much more on this subject.) In contrast to this linear conception, the basic organizing principle of traditional gamelan musical forms is cyclical. Cyclical forms are based on a constant repetition, without interruptions, of a basic musical idea. In traditional gamelan music, the repetition is so constant that the last note of each cycle is also the first note of the next cycle. (Modern pieces may have a freer form.)

Of course, music that is only repetitive becomes boring quickly, so during each cycle some things will remain the same while others change. Much traditional gamelan music consists of colotomic cycles, in which the colotomic instruments (usually gongs) play the same exact basic pattern within each cycle, while other instruments play a melody that varies from one cycle to the next. As a simple example, a form might be based on eight beats, with a large gong playing on beats four and eight of every cycle, and a smaller gong playing on beats five and seven. Some forms have a cycle even simpler than this; others can be very long, with a complex colotomic pattern playing out over a cycle that is more than 200 beats long. Many pieces (with different melodies) may be based on the same form (colotomic pattern), and pieces based on a traditional form will include the name of the form in the piece's title.

Typically, the pattern is repeated many times in the piece, forming a continuous cycle, with strokes from the larger gongs marking the most important beats (for example, that crucial last/first beat of the cycle). A piece will also have a basic melody, and some form of the melody will also occur in each cycle. The unvarying colotomic pattern, along with the basic melodic idea, provides the stability for the music, while interest and variety are provided by the many, often complex, variations on the melody.

Some of the many variations on the melody will sound quite similar to each other; others will sound quite different. The program for when, how often, and how many times, to play each variation may be set in advance and memorized by the entire ensemble, or may be signaled by the group's leader (often a drummer), or may be a combination of planned and signaled. Particular variations may be closely coordinated with events in the dance, theater production, or religious ceremony.

A wide variety of percussion instruments (see Balinese Gamelan) provide the typical timbres, although quieter ensembles may feature thin, florid lines from flutes and bowed strings. Listen for the long-lasting resonance of large gongs and low-pitched metallophones, the bright metallic sound of high-pitched metallophones and hollowed ring of smaller kettle-gongs, the clatter of cymbals, and an intricate variety of sounds from the drums.

Traditional Balinese music prefers textures that are bright, thick, and busy, often very loud and "noisy" by Western standards, with wide tuning and many simultaneous layers of sound, quite unlike any textures commonly found in Western music. Polyphony is not used. Monophony is common, but with a wide tuning. Functional harmony is not used, but the multiple simultaneous versions of the melody can often produce an impression of harmony, since more than one pitch may be sounding at a time. These multiple layers can be considered a kind of heterophony, but the resulting texture is still very different from the types of heterophony found in musics from other areas (such as middle eastern).

The texture of the typical gamelan sound is based on melodic layers. The fundamental layer is, of course, the colotomic instruments playing the form. Just "above" this layer are lower-pitched instruments playing the most basic version of the melody. Increasingly higher-pitched instruments play increasingly complex, ornamented versions of the melody. Balinese "ornament" notes during a solo may be similar to those in Western and middle-eastern musics, which tend to be free-flowing and indefinite in length, and noticeably shorter than the notes of the main melody. When the entire ensemble is playing, however, the "ornament notes" in the most complex parts tend to be very rhythmically precise, so that the total effect of all of the parts is an impression of a very constant, steady pulse.

Rhythm is a very important aspect of the Balinese musical aesthetic. A good ensemble is one that can play very complex rhythms very fast, with a very tight, clean, "together", ensemble sound. Although the ensemble rhythm is usually a very steady, even flow of notes, the technique used to get this steady, even flow at such fast tempos involves having individual players play parts that do not have a steady, even, flow, and in fact are often highly syncopated.

One way this is often achieved is by having players of similar instruments play the same syncopated rhythm, beginning on different beats. For example, if a cymbal part in an eight-beat cycle consists of notes on beat 1, 4, and 7, three cymbal players, beginning the pattern on beats one, two, and three respectively, are enough to give a beat that is steady and continuous, and yet changes timbre (and sometimes pitch, too) from one beat to the next.

Figure 1: Three percussionists playing the same syncopated rhythm, offset by one beat, can produce a sound that is a steady stream of beats with varying timbre.

This is not the only way that such closely cooperative parts are produced. In the technique called kotekan, two distinctly different parts are combined, again to form a line that the ear hears as a single part. In a typical gamelan gong kebyar, for example, the elaborated-melody part for the highest gender instrument may have two, four, or even eight notes for every note of the core melody. For variety, some variations of the melody will be slower, while others are faster. At the fastest speeds, this part may be literally unplayable for a single person, so for these sections it is divided into two parts. The polos part is based on the melody, and still plays the same note as the core instruments at important points in the melody. The sangsih part may sometimes play at the same time, but also fills in gaps in the rhythm of the polos. At slow speeds, players may play both parts, but at high speeds, each player plays only one of the two interdependent, interlocking parts. Each part is incomplete by itself, and may be highly syncopated; when played together correctly they form a single steady stream of notes.

Figure 2: A typical kotekan forms a steady stream produced by the interlocking polos and sangsih parts. Note that the individual parts may be very syncopated.

Even when the main rhythm of a gamelan piece is a very steady stream of notes, the rhythm will sometimes be interrupted by dramatic cadences marking important points in the piece (which often correspond to actions in the dance or play that the music accompanies). Changes in tempo and dynamics also add to the drama of the music, and all of these are determined (sometimes during the performance, in coordination with the dance or drama) by the leader of the group, usually a drummer, who indicates them both with gestures and with drummed signals. The ensemble that can maintain a very "clean", highly-coordinated sound during such changes wins the respect of a Balinese audience.