  The center-of-gravity rule is tested for heavy and light-quark mesons. In the
heavy-meson sector, the rule is excellently satisfied. In the light-quark
sector, the rule suggests that the $a_0(980)$ could be the spin-partner of
$a_2(1320)$, $a_1(1260)$, and $b_1(1235)$; $f_0(500)$ the spin-partner of
$f_2(1270)$, $f_1(1285)$, and $h_1(1170)$; and $f_0(980)$ the spin-partner of
$f_2'(1525)$, $f_1(1420)$, and $h_1(1415)$. From the decay and the production
of light scalar mesons we find a consistent mixing angle $\theta^{\rm
s}=(14\pm4)^\circ$. We conclude that $f_0(980)$ is likely "octet-like" in SU(3)
with a slightly larger $s\bar s$ content and $f_0(500)$ is SU(3) "singlet-like"
with a larger $n\bar n$ component. The $a_0(1450)$, $K^*_0(1430)$, $f_0(1500)$
and $f_0(1370)$ are suggested as nonet of radial excitations. The scalar
glueball is discussed as part of the wave function of scalar isoscalar mesons
and not as additional "intruder". It seems not to cause supernumerosity.
