The main instrument of Pireas-style Rebetiko, without a doubt, is the bouzouki. Originally developed from the tamboura and saz and various other lute-like instruments, the first bouzoukis had six strings. Since these are in three pairs, this type of bouzouki is called a trichordo or trixordo. (That's Greek for three strings.)
Very early trichordos had movable frets, tied at the back of the neck, so that the player could adjust the notes that were playable to fit the makam or scale of the piece being played. However, as Rebetiko developed, it moved away from the classical Ottoman makam based music, and settled on its own versions of the scales. These often have the same name as the Ottoman makam, but are subtly different. The bouzouki then became a fixed fret instrument.
There is some debate as to whose idea it was to add a fourth course of strings to
the bouzouki, and change its tuning to match the top four strings of the guitar. There
is no doubt at all that the player who popularised this version of the instrument was
Not everyone regards the change as an improvement....
Chiotis was also a very early player of amplified bouzoukis, which were perhaps necessary given the size of audience he was getting. The tetrachordo bouzouki, fed through reverberation units, effects units and into powerful amplifiers, is more or less the de-facto standard nowadays, along with the Irish bouzouki (also usually amplified), and a quick Google search will get you as much information about these instruments as you wish, possibly more. Recent developments have included a MIDI bouzouki which could be used to control whole racks of synthesizer equipment. The web site for this has now vanished, so perhaps it wasn't selling.
My personal preference is for acoustic trichordo, so if you want to find more information about tetrachordo bouzoukis, try here.
Acoustic trichordo bouzouki
My own bouzouki looks very plain. This is because the luthier who made it, Karolos Tsakirian, built it as a re-creation of a bouzouki his grandfather built for Markos Vamvakaris. The bowl is a little smaller than that of a modern bouzouki, and was made on the same mould as Vamvakaris's bouzouki. The scratch plate inlays on the sound board are made of rosewood. I love the fact that it isn't covered in flashy mother of pearl, and that the sound has been improving ever since I bought it.
Of course, most bouzoukis are decorated a lot more than mine. You can see some wonderful workmanship if you search the internet carefully. The thing is, ornamentation does not make a bouzouki sound any better, and the sound is the important thing to me.
The strings of a trichordo bouzouki are tuned like this...
Note that this diagram shows the notes an octave higher than the actual notes, to avoid having a lot of messy leger lines.
Here is a recording to assist anyone who needs to tune a trichordo bouzouki.
Of course, there is more than one way to tune a trichordo. The various alternative tunings are called "douzenia", and these varied tunings can make it easier to play in a particular dromos.
Buying a bouzouki
If at all possible, go to a dealer or a luthier, and take somebody who knows bouzoukis with you if you are inexperienced. Take your time. Reputable sellers want you to be sure you are happy with the instrument, and will be happy to let you take your time.
You really should not buy any musical instrument unless you can try it out and see if you like the sound. You should also be looking at the standard of construction, of course. Every sensible author will tell you these things. As it happens, I didn't follow this advice either time I bought a bouzouki.
The first one I bought was what is called "student quality". These are made in factories, and frankly, there's nothing wrong with most of them. Most of the people who buy student quality bouzoukis on eBay, for instance, are quite happy with their purchase. My student quality bouzouki was fine, although I actually bought it by e-mail order from a firm in Santorini who will ship to pretty well anywhere, and when I sold it on the buyer was happy with it too.
There are also bouzoukis of a higher standard available on eBay from time to time. They are also advertised on all sorts of other web sites, of course. Obviously you are hardly ever going to be able to try out a bouzouki that is sold on eBay. You need to be sure you are buying from a reputable seller, so check the feeback they have, and ask questions about the instrument. If you don't get answers, be suspicious. You can see the instrument I found on eBay further up the page. I was lucky, it's wonderful. Do as I say, not as I do!