There are many different façade treatments in the Chicago six-flat (as with all the other types too). They vary by era, class of building, builder, etc...
We'll start with some elderly façades - well, older anyway - which were typically graystones. I would assume that the term greystone comes from the what I presume to be Indiana limestone, which I guess was the most fashionable material at that time.
Here are two examples of this, both with bay windows, one with (non-original) balconies, which I suspect is somewhat newer and an older one without.
The second example has a very grand, formal, front entrance which is highly ornamented in a classical manner. The entrance is central, taking equal space from each of the first floor apartments. It has also had its, most likely tin, cornice removed. The first example has retained it's stone cornice, but lost it's original balcony railings and fascia. It has a side entrance, which steals a bedroom from one of the first floor units. Both have internal staircases, lit either from above by a skylight or from the rear via a lightwell.
Then we progress on to what I call a "flat-front" façade, with inset balconies at the front. The balconies are "internal" in that they form a room within the face of the building, usually with a bedroom fronting onto it, with a side door from the living room onto the balcony. This is a case where the living room has the short end to the street, as opposed to the long side to the street - which only allows one room to the street. This allows a bedroom to share the orientation of the living room to the street, and may make the overall depth of the building somewhat shorter. This example has deep red brick (I will have to touch on color in a future post, but it has to be said that there are many different brick colors that were used) with colonial trim, and has retained what appear to be it's original balcony railings, dividers and thin stone cornice.
There is also a typical variation on the storied Chicago window, with a wider, operable window in the center of the three window unit. This building has luckily retained it's original mullion patterns as well.
I am going to leave it here for tonight; tomorrow or the next day, I will get into the next type, the sunroom front, which has one of the most typical, characteristic of Chicago vernacular details, the front sunroom. But to whet your appetites, I am going to include one which is somewhat, somewhere in between the characteristic and typical sunroom front and the flat front. This example has the sunrooms merged into a central bay housing the front stairs and vestibule forming a sort of half-sunroom on the front elevation.
Needless to say there are many more front treatments and typologies to be explored. The sunroom has huge numbers of wonderful examples, and then there are the custom and higher end buildings, with classical, architect designed features which evolved into the few, but not, luckily, too rare art deco buildings which can be found here and there.