In complex sentences it often happens that one Subordinate clause is dependent on another Subordinate clause. To show how this works it will be best to give an example:
(1) The unfortunate man had not long lain in the cavern (a) before he heard a dreadful noise, (b) which seemed to be the roar of some wild beast, and frightened him very much.
(2) A merchant, who had much property to self, caused all his goods to be conveyed on camels, as there was no railway in that country.
(3) In sentence (1) the clause “before he heard a dreadful noise” is an adverb-clause qualifying the Finite verb “had lain”, which occurs in the Main clause: it is therefore Subordinate to the Main clause in the first degree. In the same sentence the clauses “which seemed to be the roar of some wild beast, and frightened him very much”, are adjective-clauses qualifying the noun “noise”, which occurs in a Subordinate clause; they are therefore Subordinate to the main clause in the second degree.
Now take sentence (2). The clause “who had much property to sell” is an adjective-clause qualifying the noun “merchant”, which occurs in the Main clause. It is therefore Subordinate to the Main clause in the first degree. In the same sentence the clause “as there was no railway in that country”, is an adverb-clause qualifying the Finite verb “caused”, which occurs in the Main clause. It is therefore Subordinate to the Main clause in the first degree.