Tailored prototyping refers to an emerging process for prototyping software applications, emphasizing a disciplined experimental approach in order for developers to obtain an understanding of system characteristics before committing to costly design decisions. In our approach, the design of software constituting prototype apparatus is driven by experimental hypotheses concerning risk, rather than an application's functional requirements. This paper describes the principles behind tailored prototyping, then illustrates them in concrete terms by describing their application in a pilot project. The pilot used in our illustration is a parallel I/O service --- a mechanism designed to deliver pages, in parallel, from a cluster of distributed disks. The performance results show that this parallel I/O system can, in certain circumstances, deliver higher page throughput from multiple remote disks, than with a single local disk. The pilot project exemplifies our prototyping method which is applicable to a wide variety software prototyping activities. (Also cross-referenced as UMIACS-TR-95-18)