In the following two-part article Tom Nemeth expounds the basic theory underlying computers of all kinds and our centre pages reproduce the key disc of a computer designed for the Standard Libelle at an all up weight of 660 Ibs. for a map on the 1: 100,000 scale (Wac chart). Pilots wishing to make their own computers may copy this diagram, and others which will appear in future issues, but it should be understood that Libelles flown at other wing loadings will require certain changes which will also be described in due course. For most practical purposes the computer will be suitable for use in any modern standard class glassfibre sailplane. Practical advice on the use of the computer in the cockpit will also be given.