NN connects number words together in series. The last word in the
series then connects to the rest of the sentence - either
making a D connection to a noun; an S, O, J, or other
connection to a verb (in which case the number expression acts
as a noun phrase); or an ND connection.

	  +--NN-+---NN---+--NN---+---D---+
	  |     |        |       |       |
	Four hundred thousand million people live here

Numerals and single-word numbers under 100 ("3", "1000",
"three", "eighty") have NN+, disjoined with "D+ or ND+ or
S+...". Thus they must either connect forward to another
number word, or serve as the head of the number expression;
they cannot connect backward to another number word. The words
"hundred" and "thousand" have "(NN- & NN+) or ({NN-} & (ND+ or
Dmc+ or S+...)".  They must connect backwards to another
number; they may either connect forwards to another number or
serve as the head of the expression. This yields

	Four people live here
	*Four three six people live here
	*Million four people live here
	*Million four hundred people live here
	*Million people live here
	Four hundred million people live here

Whatever number word is serving as the head of the expression may
also make a DD connection back to a definite or possessive
determiner: "THE four hundred MILLION people living in New York 
will suffer." See "DD". 

However, numbers can be assembled in an additive fashion: "the fifty
five gallon drum", "It's a hundred and two in the shade". See "NA"