The radius of a complex number, written in polar form, is zero only when both the real part and the imaginary part of the number are zero, and conversely. It therefore serves as an absolute value, or norm, symbolized in the usual manner as |*z*|.

|x+iy| |
= | (11) |

The factorization

x^{2}+y^{2} |
= (x+iy)(x-iy). |
(12) |

prompts the introduction of the

= | x - iy. |
(13) |

It thereby follows that .

The angle
in the polar decomposition of a complex number is called its *phase*phase, or sometimes its *amplitude*amplitude, but beware that amplitude does not mean magnitude, unless magnitude refers to the angular distance of the point representing the complex number from the real axis. On the other hand, modulus does mean magnitude.

The absolute value is multiplicative, which can be checked by comparing the algebraic expressions involved; in other words
|*wz*|=|*w*||*z*|. It also obeys the triangle inequality,
.
Amongst other things, those two properties are sufficient to check the convergence of the sine, cosine, and exponential series for complex numbers, to verify Euler's formula. It would
still be premature to interpret the coefficients in the Taylor's series as derivatives, however.