Schmincke Aero Color Liners are empty marker pens that can be filled with Schmincke Aero Color Professional Finest Acrylic Ink, which is a liquid acrylic paint, and used to draw lines of various widths on your painting. The tips are a solid felt that won’t dry out and go hard as long as the lid is replaced. So you can draw with real paint! The markers come in six different sizes of tips .8mm to 15mm. The different size tips each require a different size body to fit onto, so each body is a different size. The 1mm width tip comes with a regular length body (Liner No.2) or an extra short body length for fitting into a small space (Liner No.4). You can buy replacement tips if you wear yours out. The labels of the pens include a small blank block in which you can note the number of the colour you have filled it with.