The good thing about pentatonic scales is that the notes in them are completely diatonic for every chord in a given key. For a bit of an example, if someone is playing an Em chord and you happen to play a D note at the same time, then to the listener the D note combines with the sound of the Em chord and they hear an Em7 tonality, which sounds cool.

Now let's say you're in the key of C. The basic chords (triads) are: C Dm Em F G Am Bmb5. The C scale goes: C D E F G A B. Now if you play lead from the C scale over a chord progression in the key of C, chances are that you'll hit some notes that don't quite sound right. For example, if we have an Em chord in our progression and you happen to hit an F note while playing lead, it will combine sonically to form an Emb9 chord, which is gonna sound kinda nasty in a day-to-day diatonic progression. Every chord in the key of C, with the exception of the C and Am chords, will have one note that transforms them into something that would be non-diatonic in the key of that chord. In other words, it would sound funny.

Hmm, I think it's too early in the day for me to be posting theory stuff. :eek: Lol!

Anyways. The good thing about playing from the C major pentatonic scale over a C major progression is that we avoid all of those potentially nasty notes we find in our straight-up C scale. The C pentatonic has these notes: C D E G A. If we add those notes to any of our chords from the key of C we get chords like Cadd9, Dmadd9 and Em6--which are pleasant-sounding chords--instead of weird entities like Dm#13 and Emb9 (chords only a weirdo jazz musician could love ;) ).

So that's the best reason for beginners to learn pentatonic scales to start with; they don't have to deal with the out-of-key sounding notes that sometimes crop up in playing lead right out of a full scale or mode.