  Many of the extragalactic sources detected in $\gamma$ rays at TeV energies
are BL Lac objects. In particular, they belong to the subclass of ``high
frequency peaked BL Lacs" (HBLs), as their spectral energy distributions
exhibit a first peak in the X-ray band. At a closer look, their X-ray spectra
appear to be generally curved into a log-parabolic shape. In a previous
investigation of Mrk 421, two correlations were found between the spectral
parameters. One involves the height $S_p$ increasing with the position $E_p$ of
the first peak; this was interpreted as a signature of synchrotron emission
from relativistic electrons. The other involves the curvature parameter $b$
decreasing as $E_p$ increases; this points toward statistical/stochastic
acceleration processes for the emitting electrons. We analyse X-ray spectra of
several TeV HBLs to pinpoint their behaviours in the $E_p-S_p$ and $E_p-b$
planes and to compare them with Mrk 421. We perfom X-ray spectral analyses of a
sample of 15 BL Lacs. We report the whole set of observations obtained with the
\sax, \xmm and \swf satellites between 29/06/96 and 07/04/07. We focus on five
sources (PKS 0548-322, 1H 1426+418, Mrk 501, 1ES 1959+650, PKS2155-304) whose
X-ray observations warrant detailed searching of correlations or trends. Within
our database, we find that four out of five sources, namely PKS 0548-322, 1H
1426+418, Mrk 501 and 1ES 1959+650, follow similar trends as Mrk 421 in the
$E_p-S_p$ plane, while PKS 2155-304 differs. As for the $E_p-b$ plane, all TeV
HBLs follow a similar behaviour. The trends exhibited by Mrk 421 appear to be
shared by several TeV HBLs, such as to warrant discussing predictions from the
X-ray spectral evolution to that of TeV emissions.
