Subordination is a relation that holds between two units of unequal status.
In many cases the type of relation is signalled by the presence of a function word, a co-ordinator or a subordinator. A subordinator is placed in first position in the subclause. Not all semantic relations can be signalled by all three types of expression, however. Linking across sentence boundaries It is sometimes claimed that subordinate clauses cannot form sentences on their own, and that co-ordinating conjunctions should not be placed first in the sentence. These are prescriptive rules, which you may do well to observe in your own writing, but they definitely do not cover what native speakers of English do in their own writing.