I was working on a (fascinating!) book last week, and it started me thinking about transitional sentences--how we use them, what they're for, and why we might misplace them.
The general rule is to place transitional sentences at the beginning of the paragraph they're transitioning toward. This gives your reader a bridge into new material by greasing the slide with the old, familiar material. It also offers a bit of a recap of prior material, which helps your reader orient themselves within the larger structure of the piece: a kind of implicit signpost. And it helps the reader see the connection between the two paragraphs; while there is an implicit connection made simply by virtue of their proximity and juxtaposition, laying out how the two sets of ideas connect in that first sentence helps strengthen the connection in the reader's mind.
But you could do all these things with transitional sentences placed at the end of the paragraph you're transitioning away from. Why don't we do this?
Well, sometimes we do--in writing, form follows function, and sometimes this is a perfectly fine idea. But more often, sticking a transitional sentence at the end of a paragraph damages your pace and your reader's sense of satisfaction. As George Gopen and Joseph Williams have pointed out, the last sentence of a paragraph is a position of strong structural emphasis. To fill that position with a sentence that leans forward makes your reader rush into the next paragraph; it snatches away from your reader the moment of pause, the exhale that should naturally occur with a sense of closure achieved.
More problematically, putting the transitional sentence in this position also upstages the material that deserves that emphatic close: the capper sentence. Whatever form your capper takes (a summary of what went before, a clever play on words, or a mind-expanding gesture to another, larger idea), it deserves its own spotlight. Don't let the transitional sentence--an ensemble player by nature--step into that spot.