Pentel Pointliner .5mm

A 0.5mm fineliner with a plastic nib built to hold its point instead of splitting or wearing down, and pigment black ink that's water- and fade-resistant, so a watercolor wash over the top won't drag the line into the paint. Consistent flow, no skipping, whether the mark is a single line or a field of cross-hatching.

This is the pen for line drawing that has to survive whatever comes after it: ink, wash, time on a wall in direct light. Precise enough for stippling, tough enough not to be the weak point in the drawing.