Pen refills vary in two major ways; the shape and size and the writing medium.
Many, but not all refills conform to international standard sizes, so that many of the brands are interchangeable. However, just because they are the same shape and size, doesn't mean that they are the same quality, or will give the same writing experience. Some manufacturers produce refills which are unique to them and will only fit their pens, while others produce a mixture with some refills unique to their pens and also some generic sizes too.
Ballpoint pens are generally retractable and have no cap. Because the point is always exposed, ball pens use an ink that doesn't dry in the point. This is usually an oil-based ink, but ballpoint pen refills are also available as 'gel' refills which use a water-based ink with a thickener added. Gel refills tend to put a lot of ink on the page and so don't tend to last as long as those with oil-based ink.
Rollerball pens use refills with a water-based ink and generally come with a cap to prevent the refill drying out. Rollerball refills give an effect that is closer to that of a fountain pen than do ball pen refills.
To confuse the issue further, 'capless' rollerball refills are also now available, which can be used in a retractable pen, but which don't dry out.
There are not too many fibre tip and fineliner refills available. They generally use a water-based ink and come in the same size and shape as some rollerball refills, so that they can be used as a direct replacement for them. The points of these refills can be quite delicate and need to be treated with care.
A selection of suitable refills is shown with each of our pens.