For my pencils (underdrawing), I use a black Col-Erase pencil to put down extremely loose lines, giving me a sense of my composition, and sometimes (as is the case here) a grid of sorts for the ground plane. I tighten it up just a little bit, and then I go over those pencils with Faber Castell PITT pens, using all of the sizes but mostly XS.
I wanted to scan this one before I erase the pencil marks, which I do so that when I scan this to blow it up and print it out in light blue it'll be easier to decipher my own intent for the inks, especially since I'll be laying them down much later. I wanted to save this one, though, because I did far more with the pencil than I usually do at this stage. Back to work! I want to see if I can finish this, the third page for today.