TACHS: Main idea in an informational passage: find the point the facts serve
Learn to find the main idea of a fact-filled passage by locating the general statement the examples support, and to keep detail, term, comparison and main idea questions separate.
When a passage is packed with facts, how do you tell which statement is the main idea?
Short answer
Look for the general statement, often in the last paragraph, that explains why the writer included the facts. The main idea covers all the examples, while wrong choices state one detail, twist one, or stretch it with an absolute word.
Know first: Telling an example from a general statement, Finding a definition in a sentence
Step 1 of 5
The facts are there to support one bigger point
Informational passages are full of specific facts: numbers, names, examples. Those facts serve a bigger point. The main idea is the claim that explains why the writer included them.
In explanations, the clearest statement of that point often comes at the end, after a phrase like the important difference is or what matters is. The first sentence often just defines a term.
| Question | What a right answer looks like |
|---|---|
| Detail | Narrow: one fact, stated in one place |
| Term in the passage | The definition the passage gives, not a general dictionary meaning |
| Why the author compares | The comparison makes an unfamiliar idea easier to picture |
| Inference from the end | Follows from the general statement, with no extra claims |
| Main idea | Wide: covers every example in the passage |
A detail can be a perfect answer to a detail question and a wrong answer to a main idea question.
Step 2 of 5
Let the last sentence gather the examples
Short and long signals
Morse code turns each letter into a pattern of short and long signals.1 The same pattern can travel as clicks on a telegraph wire, flashes from a ship's lamp, or taps on a metal pipe2. A sailor watching a light and a clerk listening to a wire can receive the very same message. What matters is the pattern itself, whatever carries it.3
- Opening: a definition.
- Examples: three different carriers.
- The general point the examples support.
Worked example: What is the excerpt mainly about?
Which statement best expresses the main idea of the Morse code excerpt?
- AMorse code was used only by sailors.
- BThe same coded patterns can carry a message through very different signals.
- CLamps send messages more reliably than wires.
- DMorse code uses short and long signals.
- 1
First and last
First: what Morse code is. Last: What matters is the pattern itself.
- 2
Check the examples
Wire, lamp, pipe, sailor, clerk. All show one pattern traveling in different ways.
- 3
See why the other choices are there
A uses only and ignores the clerk. C compares reliability, which the excerpt never does. D is true but covers only the first sentence, so it's a detail.
Answer
The same coded patterns can carry a message through very different signals.
Find the wrong step
A student answers the same main idea question.
- 1
Reads the first sentence carefully.
- 2
Decides the first sentence must be the main idea because it introduces the topic.
What went wrong
The first sentence defines Morse code. The point the examples build toward comes in the last sentence.
- 3
Picks Morse code uses short and long signals.
The fix
Read the last sentence and check it against the examples. The main idea is the same patterns can carry a message through very different signals.
Step 3 of 5
Find the point and the purpose of an example
Read this excerpt
A map is not a tiny copy of a place. It leaves most things out. A hiking map may show trails and streams but skip every building. A subway map may bend real streets into straight lines so riders can follow the stops. Each map keeps only what its users need, so the best map depends on the trip you are taking.
Check yourself · Question 1
Which statement best expresses the main idea of the excerpt?
Answer: C
The last sentence states the point: each map keeps only what its users need. The hiking and subway maps are examples of it.
Trap answer: Subway maps bend real streets into straight lines.
Vivid detail
- Why it looks right
- It's the most surprising fact in the excerpt, and it's stated exactly.
- Why it's wrong
- It describes one kind of map. The main idea has to cover the hiking map too, and the last sentence's general point.
- The right answer
- Maps keep only the details their users need, so maps of one place can differ.
Check yourself · Question 2
Why does the author mention the subway map?
Answer: D
The subway map bends streets so riders can follow the stops. It's an example of a map changing what it shows to fit its users.
Common mistake
I picked the choice that defines the topic, because it was the first thing the passage said.
- Why it's tempting
- The opening sentence is clear and accurate, so it feels like the summary.
- Do this instead
- Opening sentences often define. Check the last paragraph for a general point, and test it against every example.
Step 4 of 5
What to remember
Remember
- The main idea is the general point that explains why the facts are there, often stated near the end.
- A detail answers a detail question, but it's too narrow for a main idea question.
- A comparison to something familiar is there to make an idea clearer, not to be the main point.
Step 5 of 5
Practice on a real question
Use what you just learned on this question, then check the explanation.
In your own words
In the practice question, which sentence of the passage states the general point, and which wrong choice is a single detail?