Brush pens present much more variation in line width than the usual fineliner pen. Their nibs have a large base that then tapers to the good point. Commonly the nib is crafted from a conical bit of felt, but there are pens which have a real brush suggestion made https://how-to-draw-with-ink-pens66533.howeweb.com/35809587/the-definitive-guide-to-how-to-draw-with-ink-pens