Electronic keyboards grammar...

Mara, to hear a Moog synthesizer listen to Switched on Bach by Walter Carlos. To hear a Hammond electric organ listen to In Memory of Elizabeth Reed by the Allman Brothers. Other than the way each sounds, I'm not sure what kind of explanation you seek.

Here is another example: to hear an Oberheim synthesizer listen to Birdland from Weather Report's Heavy Weather. If I stumble across a clear example of a Fender Rhodes electric piano I'll post it too.

Here's some general information I've gleaned from watching and listening to keyboard players "operate" their instruments: real pipe organs use stops to imitate other real instruments like oboe or french horn. Organs can play chords, multiple notes sounding simultaneously. Electric organs imitate pipe ones. The earliest synthesizers could not play chords, only individual notes. The Oberheim offered polyphony but a limited number of sounds. Today's synthesizers are mated to samplers and connected to musical instrument digital interfaces (MIDIs). For example now we can sample the sound of you snoring, the howl of Hendrix's strat or Humphrey Bogart's voice and save it to a floppy disc. In short, any sound can be used as a foundation to create the entire musical scale.

Rockvirgo,
You've got my point.
I want to know differences not only sonic but someway structural as well.
Can't that be just one synthesizer that can sound as all of Hammonds, Rhodes, Oberheim etc... or those mentioned have a very specific and unrepeatable sound?

Mara, pianos differ from organs because when you strike a piano key it hammers a string and bounces off. If you want to hear that note again you must restrike the key. In an organ the note sustains itself for as long as the key remains depressed. A Boesendorfer piano has a unique sound due to a complex combination of hammers, felts, sound boards and strings. The grand church organ in Salzburg sounded as it did due to a unique combination of bellows, organ pipes, and the church's acoustics.

The earlier electronic keyboards each have a unique sound because that was the only sound built into them. Looking backwards, they were limited by what the manufacturers could or did make available. The Hammond organ makes a special sound because its electronic circuits produce unique loudspeaker oscillations. When the Oberheim came out I believe the factory offered a limited number of plug-in sound cards. So although samples can mimic any sound, samples cannot replace the feel, sensation and technique of playing the original instrument.

Synthesizers are a huge and complex subject. As a starting point I would recommend this book.

There are many different types of synthesizers and and it is possible for a single synth to provide the sounds of virtually all other instruments. It's simple a question of how much DSP power and memory you want to put into a single machine. In essence a synthesizer is a sound generating oscillator followed by filters (tone controls) and waveform modifiers (attack, sustain and decay). Popular synth models combine multiply oscillators with an internal mixer and effects (reverb, EQ, compressors, etc.) groups. Individual instrument sounds are created by programming multiple oscillators, mods and effects signal pathways and are called patches. Ironically, one of the most common uses of contemporary synths is to emulate the sound of the so-called classic synths.