Pens for line and wash (from left): dip pen, stylus technical pen, water-soluble felt tip, permanent marker, brush pen (bottom left) and twig dipped in ink (bottom right)
Line and wash is the addition of watercolour or dilute ink to a drawn line, usually with a pen. It has been advocated and used since the Renaissance as a useful way of working, usually to complete studies prior to an oil or tempera painting.
Line and wash has always been the favoured way of working outside, because tonal paintings can be created quickly and worked from later in colour.