Even $n^{\text{th}}$ roots are defined only for non-negative real numbers. When $n$ is even and the radicand is positive, two real roots exist; the positive one is known as the principal root. For example, the number $16$ has two square roots. The principal root of $16$ is $4.$ In contrast, when considering odd roots, such as cube roots, there is only one real root, which then is the principal root. Since odd roots are defined for all real numbers, the principal root can be either negative or positive. Suppose that $n$ is an integer greater than $1$ and $a$ is a real number.

	$n$ is even	$n$ is odd
$a>0$	Two square roots: one positive and one negative. The principal root is the positive one.	One positive root, which is the principal root.
$a=0$	The only root is zero.	The only root is zero.
$a<0$	No real roots.	One negative root, which is the principal root.

When dealing with an even $n^{\text{th}}$root, use the principal (positive) root unless the root is applied to both sides of an equation, in which case both the positive and negative roots may be considered.