Montblanc refills are designed to fit exclusively and perfectly into Montblanc writing instruments for optimum performance. The tips of the refills are specially designed to withstand the effects of heat and humidity. The 0.5mm fits the Meisterstuck Classique and Solitaire Mechanical Pencils. The 0.7mm is designed to be used in the Meisterstuck Classique, Homage a W.A. Mozart, Solitaire, Generation, Noblesse Oblige and StarWalker Mechanical Pencils.