  We study the properties of galaxies with very thin discs using a sample of 85
objects whose stellar disc radial-to-vertical scale ratio determined from
photometric decomposition, exceeds nine. We present evidences of similarities
between the very thin disc galaxies (VTD galaxies) and low surface brightness
(LSB) disc galaxies, and conclude that both small and giant LSB galaxies may
reveal themselves as VTD, edge-on galaxies. Our VTD galaxies are mostly
bulgeless, and those with large radial scale length tend to have redder colors.
We performed spectral observations of 22 VTD galaxies with the Dual Imaging
Spectrograph on the 3.5m telescope at the Apache Point Observatory. The spectra
with good resolution (R ~ 5000) allow us to determine the distance and the
ionized gas rotation curve maximum for the galaxies. Our VTD galaxies have low
dust content, in contrast to regular disc galaxies. Apparently, VTD galaxies
reside in specific cosmological low-density environments and tend to have less
connection with filaments. Comparing a toy model that assumes marginally low
star formation in galactic discs with obtained gas kinematics data, we conclude
that there is a threshold central surface density of about 88 Mo/pc**2, which
we observe in the case of very thin, rotationally supported galactic discs.
