Here you'll find just the shorter variation of Internatinal Standard Cartridges. Most pens that use these standard cartridges can use either type, but even then there's usually a good argument for using the shorter ones. If a pen can use the longer type, it can usually also take the shorter type and a spare. With most pens, pop one of these short cartridges into the barrel, with the narrow end up, then fit the other as normal. When you run out, you have a spare with you already!
Some pens, usually small pens, can only use these shorter cartridges, only holding one at a time. Fortunately, the majority of standard cartridges available are of the shorter variety, so you won't find your ink choices limited much if you have one of these compact pens.