We also observed that there can be different sequences of productions that generate the same string. Some of these sequences differ only in the choice of which nonterminal to expand first. Parse trees are easiest to draw for context-free grammars. At the root of the tree is S, the start symbol. At the leaves of the tree are terminals, which are read from left to right.
Internal nodes are nonterminals.
The children of an internal node are the symbols at the right-hand side of a production for the nonterminal. Some examples may clarify this issue. Some grammars are ambiguous in that there is more than one parse tree for the same statement. Ambiguity is bad, particularly as the structure of a parse tree may give information about desired interpretations. The specification of a language should therefore be unambiguous.