Let's review what we've covered so far. We started with the basic element of improv which is rhythm and the five different methods used to create rhythmic diversity. Next, we began understanding a chord change starting with the letter that is written. This is the "root" of the chord. And from the root, we counted to the 5th pitch above the root to give some ideas for different pitches. Then, we learned that to make a true chord, we need to add the 3rd note above the root. There are two types of 3rds which changes whether we have a major chord versus a minor chord. Whew! Time to move on.
In orchestral music, the three note chord was the standard chord for many years. As we got to the 20th century, composers started to experiment more often with four note chords. This practice became standard in the jazz world and we haven't looked back. In fact, it is very rare that you will find a jazz composition that has a three note chord designated.
So what is the next most common pitch added? Well, let's again go back to our original chord: 
Usually in other posts, I've said to remove all the numbers and symbols. This time, we're going to remove all but one number:
We've kept the number 7. The 7th is the most common pitch used to make a 3-note chord into a 4-note chord. So, how do we figure out what pitch would be the 7th? Let's go back to our alphabet:
Now, the muscial alphabet is really more like a circle than a straight line, so I can take the "A" and "B" and move them from the left side to the far right side, like this:
Hopefully, you can figure out at this point which pitch is going to be the 7th. "B" is the 7th of a "C" chord. Now, before I continue, there is something I must try and explain concerning 7ths. There are actually two types of 7ths. The type of 7th is again determined by the how the chord is written in the music.
The first type of 7th is know as a major 7th, and the second type is the minor 7th, more commonly known as a dominant 7th. Before I go any further, I want to make sure you don't confuse major and minor chords with major and dominant 7ths. They are two distinct concepts even though they use the same words.
Major 7ths and dominant 7ths are intervals. Intervals refer to the number of half steps between two pitches. Major and minor chords are specific combinations of at least 3 pitches.
The major 7th is the 7th that occurs naturally within the root's key signature. Again, since there are no sharps or flats in the key of "C", "C" to "B" is a major 7th. So, a C major chord with a major 7th would have the following pitches:
And the chord symbol would look like this: 
If you look closely, though, you'll notice that to get the major 7th, we needed to have the "MAJ" included in the chord. We don't have that in the original chord. We have just the "C" and the number 7. So, since we don't have the necessary symbols to have the major 7th, that must mean that the chord we've been using calls for the dominant 7th.
To get the dominant, or minor, 7th, we need to take the 7th that occurs naturally and lower it by a 1/2 step. This usually means adding a flat, but as we progress, you'll find that is not always the case. So the notes of a C major chord with the dominant 7th looks like:
Now, this is a lot of information to take in, especially if you're new to improvisation. So, I want you to take your time. And if you have questions, leave a comment and I will make sure I answer as clearly as possible. Alright, have fun.