These are the keys when we place a capo on frets 1 - 9 while fretting the flat keys. C is neither a sharp nor flat key.
A guitar capo [capo = cap = head] is a clamp that is used on the guitar to make the neck shorter. By placing a capo (head) on a particular fret, we create a 'new nut'; a new zero fret.
Make sure when a capo is placed, that it is straight, & makes good contact with the fret (evenly). We can get 'buzzing' if the capo is not set well.
When we place a guitar capo at a certain fret, & play the chords of a given key, we are actually playing in a new key. We can place the capo, play in a key such as C, and it will sound as a different key. We typically think in the key that is being fingered, yet it is okay to think either way [the fingerings or the soundings - or both!]. The capo takes care of this, so it is easier to think in the key that we are fingering.
If we are jamming with other people, & recognize the key that they are playing in, we can reinforce - thicken it up, expand - a harmonic rhythm with brilliance, by using the capo, while fingering a different key. This is one way to make friends fast. We aren't 'interfering' with what is being played by someone else, but actually expanding the harmonic palette.
A capo can be used on the fretboard for any tuning & anywhere on the fretboard. One really cool thing about using them is that our guitars have 'sweet spots'. When we place a capo, say, on the 3rd fret, & play in G, our guitar resonance may just 'light up.' Listen for the sweet spots, while exploring different keys - every guitar is different.
Capos are also great tools for singers to find the right key for a song. Try a song that you sing in a different key by moving the capo to different positions. They are also great training tools for scale playing and for learning other keys.
A capo isn't mandatory, since our fingers do a similar thing as a capo when we are fretting higher versions of chords. The capo does make things easier, & gets some great sounds & mileage for the chords and keys we already use (ie. C, G, D, A, F, E, etc.).
A key is a tonal center - all other derived tones circle around key tone as the center [the home tone]. Our ear rests on this tone.
The diatonic chords (derived tones & chords within a key - across the tones of a key center) are the 'expected' chords when you are playing or writing a song in a particular key center.
The first set of diatonic chords that we learn in every key are triads. Triads are 3 tone chords built by superimposing intervals of 3rds (from the tones of the key - every tone in the key, the scale, can be a root of a chord). The qualities derived from a Major key center are I IV V Major; ii iii vi minor, & vii diminished. When we have the triads in your hands, we then add the 7 to each chord. Example in C.
Often within a song, nondiatonic tones & chords can be used for interest, tension, & voice leading. Check out this harmonic map.