  We perform polarimetry analysis of 20 active galactic nuclei (AGN) jets using
the Very Long Baseline Array (VLBA) at 1.4, 1.6, 2.2, 2.4, 4.6, 5.0, 8.1, 8.4,
and 15.4 GHz. The study allowed us to investigate linearly polarized properties
of the jets at parsec-scales: distribution of the Faraday rotation measure (RM)
and fractional polarization along the jets, Faraday effects and structure of
Faraday-corrected polarization images. Wavelength-dependence of the fractional
polarization and polarization angle is consistent with external Faraday
rotation, while some sources show internal rotation. The RM changes along the
jets, systematically increasing its value towards synchrotron self-absorbed
cores at shorter wavelengths. The highest core RM reaches 16,900 rad/m^2 in the
source rest frame for the quasar 0952+179, suggesting the presence of highly
magnetized, dense media in these regions. The typical RM of transparent jet
regions has values of an order of a hundred rad/m^2. Significant transverse
rotation measure gradients are observed in seven sources. The magnetic field in
the Faraday screen has no preferred orientation, and is observed to be random
or regular from source to source. Half of the sources show evidence for the
helical magnetic fields in their rotating magnetoionic media. At the same time
jets themselves contain large-scale, ordered magnetic fields and tend to align
its direction with the jet flow. The observed variety of polarized signatures
can be explained by a model of spine-sheath jet structure.
