To study the neutral hydrogen, we observe its 21 cm radio emission (and sometimes absorption against a strong background continuum source). The intensity of the line is proportional to the column density of the emitting or absorbing hydrogen. Of course the hydrogen is in motion and so its emission or absorption frequency we detect is Doppler-shifted to a slightly different one. Thus we have to observe over a range of thousands of frequency channels each shifted from its neighbors by a little bit. As a result we get a "data cube" - a set of hydrogen density maps of the given region of the sky as a function of its line of sight velocity relative to us. Such line of sight velocities are mostly due to the Galactic rotation. For the inner Galaxy (i. e. distances from the Galactic center less than our Sun's distance, which is about 8.5 kpc) we know that the Galactic rotation curve is roughly flat - independent of the distance from the Galactic center everything (including our Sun) moves with almost equal tangential velocities of about 220 km/s. This allows us to "translate" velocity distribution into a distribution of distances from us. However, except in special cases of motion along the line of sight, there is always the unfortunate ambiguity of whether the source is on the near or the far side of the Milky Way. Thus the matters get complicated and basically the situation is not that dissimilar from an ant trying to paint somebody's portrait while sitting on the face

The aforementioned special cases still save the day if one can say that a particular source has a velocity close to the one that a source moving along the line of sight would have. In such case we say that the source is located at the tangent point and the distance is easily determined. The problem with this approach until very recently was that the Galactic interstellar medium appeared to be diffuse and thus no "sources" could easily be defined. In diffuse gas one can't tell where "here" ends and "there" starts... Only after the Green Bank Telescope (GBT) went online, enabling a view of the hydrogen sky with unprecendented in one instrument resolution, sensitivity, and dynamic range, the situation has drastically changed. In 2002 Dr. F. Jay Lockman disovered that the interstellar medium is in fact clumpy! These "clumps" came to be known as "halo clouds" because they are most easily found in the Galactic halo, but there are indications that they are abundant in the disk too. Actually they seem to be almost everywhere. A typical cloud has a mass of a few tens of solar masses and a size of a few tens of parsecs and thus is fairly small by Galactic standards - the tangent point trick works for them pretty well! Futhermore, they act as probes allowing us to study the physical conditions in the Galactic interstellar medium.