Which Pentatonic scale to play over which chord in a major key chord progression?

Pentatonics. Here, pent. major. In key C, C major pentatonic works well. Especially over any bars with a C chord. That's particularly due to two 'avoid' notes missing. C maj. pent. has C D E G A. Missing F and B. Those avoid notes are so called because they are the ones which can easily clash when played over a C chord. Out of C maj. pent. the C E and G are already sounding in that chord, and the D and A don't present any particular discordancy.

All that's o.k., but songs don't just stay on C all through!

Let's consider the V chord - G. If it's G7, then it contains those two previously considered avoid notes! They are good to play over G(7), so are not avoid notes at this point. Ironically, the worst avoid note now is C. G A B D E and F are good.

On to the IV chord - F. Obviously, F itself is a goody, although B probably isn't. So - F G A C D work best.

You may have noticed that the notes for each are their own maj. pent., with the addition of note F over G7.

On to the minors. Dm is relative to F, so the F notes work. Em is relative to G, so the G notes (minus F) work, and Am is relative to c, so C notes work. All this paragraph is a sort of blanket - 'safety blanket' is you like.

Having said all that, I often tell students that after the 'rules' are known, any note, anywhere, any time, over any chord is possible. Andd that includes non-diatonics, so use pents as a start point, but don't be restricted to only using those. Music is pretty bland when that happens. it needs the 'wrong' notes - but used intelligently!

When you compare all the diatonic chords I ii iii IV V vi viio with the pentatonic scale on the tonic C D E G A you will notice that most of the various chords don't have all their tones available in the pentatonic scale. Chords I (C E G) and vi (A C E) are complete, but all the other chords are incomplete. Ex, you can't get the two other primary chords IV (F A C) and V (G B D), because the F and B are not in the pentatonic scale.

One approach to deal with this is workaround the missing tones and target the available tones. If you focus on the tonic and dominant of the scale it covers almost all the chords...

• C with I vi IV
• G with V iii I & viio for a V7

...notice that G is in both I and V making it a fairly "safe" tone.

You could also focus on the tonic and second scale degree, the supertonic...

• C with I vi IV
• D with ii viio V

A benefit of pairing up two target tones is it helps make nice two part phrases.

But there is another thing to consider. In pop and rock style it's common for the melody - especially improvised solo lines - to not match up perfectly with chord tones. In this style chord progressions have a tendency to repeat over and over. They don't move to new keys. From a higher level perspective such progressions just set out the tonic or tonal palette. Something like ||: I V vi IV :|| in C is a good example. Repeating that over and over is basically harmonically static. It just stays in C.

When a C pentatonic scale is used to improvise over that kind of progression the concern isn't about each tone harmonizing with every chord. It's about an interesting line and a clear sense it's a line in C. The ending of phrases and lines in the improvisation will get more care to end on a good harmonic note, most likely the tonic. The "mismatch" of tones on the interior of the improvised lines is something you can exploit. It can create dynamic tension leading to a satisfying ending on a well harmonized note.

What makes a Pentatonic scale so "useful" is that you can play it on any scale degree, not just on I chords. The Pentatonic scale has at least one note in each of the chords you listed, so it will never technically be wrong to stick purely to the scale. Now, that wouldn't make for a very interesting jam, but it is a really good starting place for learning improvisation. As you continue, you can do more interesting things, like adopting some notes from the chords that aren't in the Pentatonic every once in a while.

But that is a little besides your question, so your answer would be: the Pentatonic scale on I chord will work for every chord in your scale.