In
A kind of analog of Sard's Lemma in Algebraic Geometry , sometimes the existence of certain objects is established by showing that the construction depends on a Bertini's Theorem:
"positive number of parameters". A very elementary example is the following.
Proposition. Given any $\frac{n(n+3)}{2}$ points in $\mathbb{P}^2$, there exists at least one plane curve of degree $n$ containing them.
Proof. The equation of the general plane curve of degree $n$ can be written as
$\Sigma_{i+j+k=n} a_{ijk}x^i_0x_1^jx_2^k=0$,
so these curves form a vector space of dimension $\frac{(n+2)(n+1)}{2}$. Imposing the passage through any point we obtain one linear condition sistem |L| on the coefficientsa smooth projective variety X, so by elementary linear algebra the subspace of curves containing all its general element is smooth outside the given points has dimension at least
$\frac{(n+1)(n+2)}{2} - \frac{n(n+3)}{2}=1$.base points".
In particular, it if $|L|$ is not emptybase-point free then the set of smooth elements in $|L|$ is dense, in particular non-empty.