The layout for BNF works like:

LHS ::= RHS

LHS — (Left Hand Side)
::= — Is Defined by
RHS — (Right Hand Side)

The Right Hand Side defines the Left Hand Side

An Example being:

<DIGIT>::=1|2|3|4|5|6|7|8|9|0

This is also an example of a Terminal type where Digit can only be defined in terms of itself with no further way to break it down.

An Example of a Non-Terminal being:

<DIGIT>::=1|2|3|4|5|6|7|8|9|0 — Terminal
<LETTER>::=A|B|C|D|E|F|G|H|I|J|K|L|M|N|O|P|Q|R|S|T|U|V|W|X|Y|Z — Terminal
<SPACE>::=’ ’ — Terminal

SP10 5NL

<POSTCODE>::=<LETTER><LETTER><DIGIT><SPACE><DIGIT><DIGIT><LETTER>— Non-Terminal

| — Acts as the word ‘OR’

Recursive use:

Making use of <LETTER>::=A|B|C|D|E|F|G|H|I|J|K|L|M|N|O|P|Q|R|S|T|U|V|W|X|Y|Z

<WORD>::=<LETTER>|<LETTER><WORD>
This statement means that for WORD the choices can be either a single <LETTER> ending the recursion being the base state OR ( represented by the ‘|’ ) <LETTER> and then <WORD> again???

This extra call of <WORD> again is what starts the recursion with the process working out like if the OR statement is chosen it will then run like the first argument being simply <LETTER> to which then follows is of course <WORD> restarting the entire thing getting the crossroad of <LETTER>|<LETTER><WORD> these two being the two and only two choices available.

Syntax Diagrams
Simply put are a visual | graphical version of BNF