  In order to perform a detailed study of the stellar kinematics in the
vertical axis of bars, we obtained high signal-to-noise spectra along the major
and minor axes of the bars in a sample of 14 face-on galaxies, and used them to
determine the line of sight stellar velocity distribution, parameterized as
Gauss-Hermite series. With these data, we developed a diagnostic tool that
allows one to distinguish between recently formed and evolved bars, as well as
estimate their ages, assuming that bars form in vertically thin disks,
recognizable by low values for the vertical velocity dispersion sigma_z.
Through N-body realizations of bar unstable disk galaxies we could also check
the time scales involved in the processes which give bars an important vertical
structure. We show that sigma_z in evolved bars is roughly around 100 Km/s,
which translates to a height scale of about 1.4 Kpc, giving support to
scenarios in which bulges form through disk material. Furthermore, the bars in
our numerical simulations have values for sigma_z generally smaller than 50
Km/s even after evolving for 2 Gyr, suggesting that a slow process is
responsible for making bars as vertically thick as we observe. We verify
theoretically that the Spitzer-Schwarzschild mechanism is quantitatively able
to explain these observations if we assume that giant molecular clouds are
twice as much concentrated along the bar as in the remaining of the disk.
