Originally Posted by: spcharles24

I heard that songs can be played in different key than a song in that original key, some online softwares help me to do that job, but how to do it manually by our mind??[/quote]

Anders explains transposing in this tutorial. He uses a capo in the example, but also covers the general concept.

[quote=spcharles24]For example if a song is in the key of G how to play it in the Key of C

You have to know the musical interval distance between the notes.

From G to C is a 4th (or 5 half-steps: g-g#-a-a#-b-c).

So you move every chord up (or every chord down) that exact musical interval distance. This is where the concept of chord progressions becomes extremely valuable.

Key of G example: G(I) - Emin(vi) - C(IV) - D(V)[br][br]

Transpose the progression to C:

G up a 4th is C

E up a 4th is A

C up a 4th is F

D up a 4th is G

Key of C: C(I) - Amin(vi) - F(IV) - G(V)

Hope that helps!