Hey all 
I am new to this site and very new to trying to learn about chords and playing by ear. Over 20 years ago I use to play by reading notes and since I havent played in over 20 years (just recently picked it back up) I have forgot alot that I learned back then and I am currently refamilarizing myself with all the chords. By reading the notes, I dont remember ever really learning them, I just saw them on a sheet of music and played them. I have a problem with playing by notes because other people that play instruments (guitar, etc) never can seem to pick up by ear what I do by notes. That has made me want to re-learn things all over and try doing it by ear. I am confused and need an explannation on what "key signature," "perfect pitch," "tonic chord," and "relative pitch" are. I never knew there were so many ways. 
First of all, "Greetings and welcome to LGM, apostle5!"
I'll try and answer your question to the best of my ability...
"Key Signature" - By knowing the key signature, I can determine what key the song is in. For example,
when I see 2 sharps (#), I know the song is in the key of "D"; or if there are 5 flats (b) in
the key signature, then the song is in "Db".
"Perfect Pitch" - This is the ability to hear any note, and know what note it is. If I had perfect
pitch, you could play any note on the piano, and I would be able to tell you
what note it is, just by hearing it. This is a valuable skill to have because it allows you to
hear the notes being played or sung, and to follow the music.
"Tonic Chord" - This is the
number 1 chord of the key that is being played. By following the chord
progression, you hear the tonic chord (1 chord), and the key that the song is in, is
reflected by that chord. For example, a progression may go like this:
6-3-2-5-
1 or Bbm- Fm- Ebm7- Ab7-
Db. The tonic or 1 chord is
"Db", therefore the key of the song is "Db".
"Relative Pitch" - Relative pitch is similar to "perfect pitch" in that you can hear the note that's being
played without seeing it. The BIG difference is that, with relative pitch, you first have
to be given the name of one note or chord, and from there are able to determine the
interval of another note being played, without seeing it. For example, if you were to hit a
"D" note on the piano, and tell a person with "relative pitch" that it's a "D" note, then he
or she could then call out the other notes you hit without seeing it.
Once again, this is the best understanding I have of these methods. T-Block and others have a wealth of knowledge that they share and have posted a ton of helpful information. I encourage you to check it out.
God bless you, BroAllan
