I t had first been suggested by that eminent and redoubtable "Rebetologist" Elias Petropolis, that a good part of the origin of the Rebetiko Ethos, might be traced to mainland Greece as far back as the mid 19th century, as well as to the Greeks that lived on the west coast of pre 1922 Anatolia. Specifically it was Petropolis who first suggested that at least part of the behavioral code of the Rebetis (which strongly emphasized that sense of Independence) may have been something of a variation, that had it's origins in the attitude and lifestyle of these Klepht brigands from the 19th century. During the 19^th century, it was they who accounted for a large percentage of what was then the "criminal underclass".  It was also Petropoulos who identified and characterized three different periods for Rebetiko music.  Each of these in approximate ten year increments. At first glance this construct may seem a bit simplistic, but when reconsidered it does serve the purpose of a functional and verifiable reference point.  While it is true that the first and second periods can and do overlap, it is also true that each of these periods can be identified as such, primarily through each periods unique musical and structural  characteristics which give them a very distinct & dominant sound. The first of these was the " Smyrnaic" period, which lasted from roughly 1922-1932 and which was characterized by the Cafe Aman style of improvisation that was prevalent in many of the Asia Minor Cabarets mentioned earlier at the turn of the century. The second catagory Petropoulos called The "Classic" period. This would have been extant from around 1932-1942. This is arguably the most interesting because it is arguably the most active and productive period for the entire genre.  It was primarily this period, that defined the structural context of the music, which more or less constitutes what we normally identify today as Rebetiko.  The third category is known as the "Popular" period.  It was placed from approx 1942 to 1952 and this was the point in time, where Rebetiko music began to acquire an acceptance and respectability it had not previously managed to capture.  It's emphasis seems to be more related to the exposure and dissemination of the genre, then to the actual development of another structural style.  Petropolis basically demonstrates how the sound of Rebetiko, evolved from a sound that once displayed the very distinctive eastern influences inherently contained within it's structure, to a sound that gradually moved toward a more western influenced and dominant type of structure. This is a distinction that is readily apparent when listening to various examples of recordings from before and after the 1930's.  However one needs to keep in mind, that most of the recordings that supposedly represent the "Smyernaic" sound , were in fact made during the 1930's, when that period had for the most part already ceased to exist.   However there were a number of recordings made before 1930 , specifically in the USA and these can help serve as a frame of reference in approximating what the Smyrna sound must have been during it's reign . Generally with the Smyrniac material , one can usually discern a sound that is quite different from the later "Classic" period. The Smyrna material was often played with some instruments that were used to obtain a sound, that in the West we might best characterize as " Oriental ".  The other instruments that we would recognize, were more or less tuned in a similar fashion. Without attempting to get very technical (and considerably over my head) the structure for this type of music (as I understand it) , has a considerably larger number of secondary and tertiary notes and consequently the harmonic structures are very different and more complex then that of their western counterpart. This often results in the production of a tone that can sound alien and somewhat dissonant to the casual and untrained western ear and at times can lead to the impression, that the musicians or singers are playing or singing "off key". Their not !  In fact because of the music's inherent structural complexity, it can often be very difficult for some western trained musicians to reproduce these Modes properly. They are known as "Dromi", (which is the Greek term for roads) and also to some extent are based on the Turkish version of Modes known as Makhems.  After the population exchange many Greek musicians gradually began to experiment more and more with the "conventional" western style scales along with their different harmonics and eventually dropped the majority of the more Eastern flavored Dromi.  But not entirely ! Some musicians decided to hold on to a few they felt somehow "worked" well with the western scales. This is one of the reasons why at times, even the "Classic" period of Rebetiko can still sound a bit "Oriental".  Anyway this is one of those subjects, that can lead some down that "Yellow Brick Road" of  Ethnic/historical (where does it all come from and what does it all mean) debate. These types of issues often try and focus on the Ethnic Origins of the various Dromi.  Are they Greek are they Turkish are they Byzantine are they Arabic are they Indian are they old Persian are they this or are they that !  So much for Rebetiko's contribution to the Multi - Cultural Holy Wars of the fast fading 20th century.