2 Is the capacity of short-term memory fixed or does it improve with practice? It is already known that training on complex working memory tasks is more likely to transfer to untrained tasks with similar properties, but this approach has not been extended to the more basic short-term memory system responsible for verbal serial recall. Here we investigate this with the adaptive training algorithms widely applied in WM training. Serial recall of visually presented digits was found to improve over the course of twenty training sessions but this improvement did not extend to recall of either spoken digits or visually presented letters. In contrast, training on a non-serial visual short-term memory color change detection task did transfer to a line-orientation change detection task. We suggest that training only generates substantial transfer when the unfamiliar demands of the training activities require the development of novel routines that can then be applied to untrained versions of the same paradigm (Gathercole, Dunning, Holmes, & Norris, 2019). In contrast, serial recall of digits is fully supported by the existing verbal short-term memory system and does not require development of new routines.