The negative graduation printed on the pipettes increases the possible pipetting volume of the values nominally indicated on the pipettes. In this way, if the nominally indicated volume of the pipette is exceeded slightly it is not necessary to use the next largest serological pipette, but rather an accurate pipetting process can be carried out with the pipette currently in use. Example: you are working with a 25 ml pipette but want to transfer 32 ml of liquid. The negative graduation means that this is also possible with the 25 ml pipette accurately and without any problems.