Smith (1926) estimates, from diary data, that at 2;6 total vocabulary is just less than 500 items; by 3;0 it is almost 1,000 items (reported in Garman 1979); and by 6;0 it is around 14,000 (Templin 1957). However, these are only estimates. As Crystal (1987) has noted, we have very little real knowledge about vocabulary size, and what we have are most likely to be underestimates. On the assumption that children start building their vocabulary at around 18 months, Clark (1983) estimates that they add, on average, nine new words a day from then on. Having looked at some of the open-class words in the previous chapter, the focus of the following discussion will be on the development of some of the major closed-class (function) words.