Which ink do you use?

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I am often asked how I stop the ink running when I add watercolour to a drawing; the answer is, I use waterproof ink - a brand called 'Higgins Black Magic'.  Then I can add the watercolour afterwards without the lines getting smudged.  Sometimes I use ordinary, non-waterproof inks, perhaps because they come in different colours, or because I'd like the line to be a bit blurry.