In a compound sentence both parts are communicatively equal if they are connected by copulative and adversative coordination. If they are connected by causative and consecutive coordination, the second coordinative clause becomes rhematic (Come to Venice early in the morning and you will see a city bathed in a sea of light).
In a complex sentence the principal clause dominates sub-clauses structurally, but communicatively it is insignificant. It is a sheer introducer of sub-clauses, it expresses modality and general assessment of the information carried by a sub-clause (I returned, and saw under the sun that the race is not to the swift nor the battle to the strong, neither yet the bread to the wise, nor yet the riches to men of understanding, nor yet favor to the men of skill, but time and chance happenth to them all (Ecclesiastes). Placed in the final position sub-clauses are usually rhematic. In a complex sentence with a complement sub-clause it is the main clause which is rhematic (It is her bright joy that surprised me).A parcellated sub-clause becomes rhematized (See parcellation). In a complex sentence based on specification it is the second part which is rhematic (Life is very mixed, sir: it is not all poetry and it is not all gaiety (B.Shaw).
Within a supra-phrasal unit sentences, constituting it, are connected by different means of cohesion: coordination, subordination, specification, reiteration of all kind, anaphoric pronouns, ellipsis, the definite article, a peculiar distribution of tenses, etc. Here we distinguish two communicative parts: the topic and the comment. The topic strives to the beginning. The comment comes last carrying either new or most important information (He is the kind of young man who does everything well. He is good at games, an excellent shot, a good amateur actor and can tell a first-rate story (A. Christie).