As noted in Chapters 11 and 12, there are two common ways of obtaining design and specification of some part of the works within a main contract which is mainly a build-only contract, without the necessity of isolating the work in a nominated sub-contract, which also may depend on performance specification and has numbers of principles corresponding to those in this chapter (see Ch. 18). One is by using an arrangement such as the JCT contractor's designed portion supplement considered in Chapter 12 when the extent and separateness of the portion are quite high. The other method is that of introducing performance specified work, as mentioned in Chapter 4, which is taken in this chapter and is intended for relatively limited work. The methods are not mutually exclusive and they may be used together in the same contract. The situations for using one or the other are not conceptually exclusive either, as may be inferred from the terms 'quite high' and 'relatively limited' used in this paragraph.