Circle of fifths

[iommi]

I can't understand the circle of fifths.Around the circle There are keys like Bb which can be called A#. Can't it? Also when it is said the difference between any two notes is a perfect fifth i have to do the scale and hence i will know the no of sharps or flats in a scale.Then why should i know the circle of fifths?Can this be used to tell which note in a scale is sharpened or flatened?

In constructing major scales it was written 3 + b3rd . Can someone explain what it means with reference to c major scale and some other ?

[cyberfret]

The circle of 5ths will help you learn what the notes are in each key. It is a way to "visually" learn the relationship of the major keys.

First you will need to have an understanding of how major scales are build.
http://www.cyberfret.com/theory/major-scales/index.htm

The circle of 5ths involves key signatures, here is a lesson on major key signatures.
http://www.cyberfret.com/theory/major-key-signatures/index.htm

Here is the cycle of 5th for the major keys
http://www.cyberfret.com/theory/circle-of-5ths-major/index.htm

Understanding what a perfect 5th is will take a knowledge of intervals.
http://www.cyberfret.com/theory/intervals/index.htm

Fully understanding theory takes a little knowledge of a lot of things. So you will need to read through these lessons, and maybe some theory lessons from some other sources before you can completely put the big picture together.

There is no key of A#. The reason is that A sharp would consist of these notes A#, B#, C double sharp (raised 2 half steps) D#, E#, F double sharp, and G double sharp. The key of Bb major is Bb, C, D, Eb, G, A..... a much simpler way to think of the same sound.

As far as a perfect 4 being made up of a 3rd (major 3rd) plus a b3rd (minor third)..... from a C to an E is a major 3rd. From E to G is a minor 3rd. From C to G is a perfect 5th. That intervals lesson will help you understand this more.

--Shawn