We prove the longstanding physics conjecture that there exists a unique two-parameter
${\mathcal {W}}_{\infty }$
-algebra which is freely generated of type
${\mathcal {W}}(2,3,\ldots )$
, and generated by the weights
$2$
and
$3$
fields. Subject to some mild constraints, all vertex algebras of type
${\mathcal {W}}(2,3,\ldots , N)$
for some
$N$
can be obtained as quotients of this universal algebra. As an application, we show that for
$n\geq 3$
, the structure constants for the principal
${\mathcal {W}}$
-algebras
${\mathcal {W}}^k({\mathfrak s}{\mathfrak l}_n, f_{\text {prin}})$
are rational functions of
$k$
and
$n$
, and we classify all coincidences among the simple quotients
${\mathcal {W}}_k({\mathfrak s}{\mathfrak l}_n, f_{\text {prin}})$
for
$n\geq 2$
. We also obtain many new coincidences between
${\mathcal {W}}_k({\mathfrak s}{\mathfrak l}_n, f_{\text {prin}})$
and other vertex algebras of type
${\mathcal {W}}(2,3,\ldots , N)$
which arise as cosets of affine vertex algebras or nonprincipal
${\mathcal {W}}$
-algebras.