Topic, Main Idea and Supporting Detail
How to write the fundamental text pattern for most types of writing
Why topic sentences are important
Look on your bookshelf (or your Kindle). What do you see? You see titles (and authors). For most non-fiction books the title gives you a fairly good idea of what you will find in the book. Apart from helping you decide whether to open the book, once you do the title prepares your mind for understanding the content. The title gives you the main idea of the book. The book itself is almost always divided into chapters and the chapters themselves have titles so that you know what the main idea of each chapter is. So there is a pyramidal, hierarchical structure which is helpful in non-fiction texts.
This same pyramid structure is found at a lower level in the text itself: each paragraph usually has a topic sentence, which is the controlling, main idea for the whole of that paragraph. The rest of the paragraph is detail about this main idea. The detail may be explanation, supporting arguments, evidence, reasons, and so on.
This method of text organisation is helpful for the reader because it helps with understanding the content of each part of the text, and it shows, or hints at, where the text is leading. It's also very helpful to you, the writer, because by following this pattern it helps you to write well organised, coherent texts.
The example below shows how a topic sentence is used in an expository text and how cohesive ties in the paragraph point back to the main idea.
The last sentence in this paragraph also has three items which point back to the topic sentence: "plants", "do this", and "one or two".
"plants" is lexical repetition pointing back to the plants catch and devour the tinier animals.
"do this" refers to build traps which catch unwary insects and also further back to catch and devour the tinier animals
"one or two" is ellipsis and means one or two of these insect eating plants.
So you can see that there are plenty of connections back to the topic sentence at the beginning. You can tap or click in sentence 4 of the animation to see them all.
This last sentence in this paragraph also leads us forward into the rest of the text. We would expect to find some more information about these "one or two plants you might be able to find". In fact, the very first sentence in the next paragraph gives us an example; the sundew. There are more examples in the subsequent paragraphs.
Sometimes a topic sentence operates as a topic not only for its own paragraph but also for the following ones. This is what is happening in this section of the text. The title of this section is "Plants which Eat Insects" so it's not surprising that the first paragraph has a topic sentence which expands on this and sets the main topic for the following paragraphs. The second paragraph of this text gives an example of a plant (the Sundew) which devours tiny animals and small insects (the topic sentence). You can view paragraphs 1 and 2 by swiping left or by mousing down this page.
As a rule, plants are the sufferers and are eaten by animals, but there are cases known in which this state of things is reversed; the plants catch and devour the tinier animals and small insects such as flies. But, you may ask, how can they do that, for the insects move so quickly, and the plants are fastened by their roots to one spot. Just as a spider builds a web and then waits quietly beside it till the flies are caught, so the plants build traps which catch the unwary insects. There are not very many plants growing wild in England which do this, but there are one or two that you might be able to find.
There is the sundew, which grows among bog-moss in wet, swampy places at the edges of lakes, or on the wet patches on hillsides. It is fairly common in such places, a little distance from big towns, but it does not like smoke, so that it will not live within a few miles of London, Manchester, or any big smoky town. It is a small plant with round, reddish-coloured leaves, covered over with little fingers or tentacles each with a sparkling drop of sticky moisture at the end, so that even in the heat of the day when all the dew is dried up, the whole plant looks as though it were spangled with tiny dew-drops. Perhaps it is this cool, sparkling appearance which attracts the insects to it, but when once a fly alights on one of the leaves, its fate is sealed. The tentacles with their sticky tips bend over one by one till the fly is quite covered in by them and cannot get away. It dies, and is digested by the juices given out by the leaf, which are very much like the digestive juices of animals.
The second sentence in this paragraph asks a question: how can they do that? This question contains two pointers back to something in the first sentence: "they" refers to "plants catch and devour the tinier animals ...", and "do that" substitutes for "catch and devour the tinier animals and small insects such as flies".
They point back to the topic. These backward pointing links in a text give the text cohesion and can help you to find the topic in a paragraph. There are even more pointers in this sentence; "the insects", "the plants" and "their" roots.
These are the first two paragraphs of the section Plants Which Eat Insects. You can find the complete text of this section on the gutenberg site. You may notice that in the second paragraph reference is made to the places where the sundew grows and that it does not favour smoky towns like London or Manchester. This book was written more than a century ago when those places were a lot smokier than they are now. Nevertheless, this plant is not common and it is still not likely to be found near large cities. You can find more information about the sundew and some fine photographs on this wildlife trusts site.
The third sentence answers the question posed in the second: How do plants catch and devour the tinier animals and small insects? The writer uses a comparison with a spider. Like a spider, the plants build traps which catch the unwary insects. Again, we have pointers back to items in the topic sentence: the plants and insects.
Topic sentences are often placed at the beginning of a paragraph. This is useful for the reader; it means that we know what the paragraph is about right from the start. In this case the first part of the first sentence is an introduction to the topic. It reminds us that usually animals eat plants, but that there are a few plants where the opposite happens - in which the plants eat animals. So it is the second part of the first sentence which is the topic of this paragraph: the plants (which) catch and devour the tinier animals and small insects such as flies.
The last sentence in this paragraph also has three items which point back to the topic sentence: "plants", "do this", and "one or two". "plants" is lexical repetition pointing back to the plants catch and devour the tinier animals.
"do this" refers to build traps which catch unwary insects and also further back to catch and devour the tinier animals
"one or two" is ellipsis and means one or two of these insect eating plants. So you can see that there are plenty of connections back to the topic sentence at the beginning. You can tap or click in sentence 4 to see them all.
This last sentence in this paragraph also leads us forward into the rest of the text. We would expect to find some more information about these "one or two plants you might be able to find". In fact, the very first sentence in the next paragraph gives us an example; the sundew. There are more examples in the subsequent paragraphs.
Sometimes a topic sentence operates as a topic not only for its own paragraph but also for the following ones. This is what is happening in this section of the text. The title of this section is "Plants which Eat Insects" so it's not surprising that the first paragraph has a topic sentence which expands on this and sets the main topic for the following paragraphs. The second paragraph of this text gives an example of a plant (the Sundew) which devours tiny animals and small insects (the topic sentence). You can view paragraphs 1 and 2 by swiping left.
