TWO to 2 1/2 YEARS—SYNTACTIC SPEECH--50 small words for baby and a big bang for language development!
At one year the child communicated symbolically for the first time. But language is more than just symbols and referents.
What gives language its big edge, as a means of communication, is the convention of putting single words into strings to express ideas! This is part of what the text refers to as Duality of Patterning.
We could try to create a symbol for every thought we would like to communicate, but very soon we would find our lexicon to be unmanageable and still growing. But by combining words to express new ideas, all of a sudden, a limited vocabulary can go a very long way!
So this is that time (2 years) when the child exhibits a subconscious knowledge of the concept (part of what is called language competency) and puts two words together to express and idea! But not until they have accumulated an expressive vocabulary of at least 50 words.