🧠 Why Some Children Mix Up b, d, p, and q

Today I had the pleasure of assessing a student in my private practice, and one of the things she spoke about was how she still mixes up the letters b and d. As she worked through the activities, I could see this happening before my eyes. It’s something very common I come across in the work that I do, and I thought I might write a few notes about it as there is more to letter reversals than meets the eye.

It all starts with something called ‘mirror invariance’.

What is Mirror Invariance?

Mirror invariance is a natural process in the brain where we recognise objects as the same, even if they are flipped or rotated. There is a common graphic that often does the rounds, of a chair. The example is that a chair is still a chair whether it’s facing left, right, or upside down.

Recognising this is something our brain is hardwired to do, as it helps us to quickly recognise objects in the world without having to relearn them from every angle.

Why is it Important?

Interestingly, mirror invariance is actually essential for survival and everyday functioning. When we are babies, we can recognise our parents’ faces from any angle. And a lion will always be a lion, no matter which way it is facing, which will trigger danger and give a warning for us!

Essentially, the mirror invariance allows us to:

• Recognise faces from different angles

• Identify objects quickly in changing environments

• Make sense of the world without needing to relearn everything

Without it, the world would feel slow and confusing as we would need to learn to see and understand things from every angle. 

Stanislas Dehaene and colleagues wrote a key paper called ‘Breaking the symmetry: Mirror discrimination for single letters but not for pictures in the Visual Word Form Area’. They discuss how the brain needs to override this built-in function in order to learn to read. Whilst it is really helpful for objects, it’s not such a helpful process for letters. 

Why Does This Affect b, d, p and q?

When we enter the world of letters and sounds, this is where it becomes more challenging for our brains, as letters are not like real-world objects. b, d, p, and q are mirror images of each other; however, in reading, the way they sit matters.

The mirror invarience reflex tells a young learner 

“These shapes look the same… they must be the same.”

But reading says:

“No, they are completely different!”

This creates a temporary conflict in the brain, which is why letter confusion is so common in our young and early readers. There are multisensory learning techniques that can help to suppress it.

When Do Children Grow Out of It?

Most children begin to resolve this confusion around ages 6–7 (Year 2–3) as they begin to build stronger connections between letters and sounds. Here in New Zealand, we are so lucky to have teachers learning to teach reading and writing the way the brain is wired to learn, so many young learners will be able to build those connections for b, d, p, and q without too much of a hindrance. 

They will begin to learn that the direction or orientation of the letter changes the meaning of what they read and write. With enough targeted repetition and good instruction, the mirror invariance around b, d, p, and q can be suppressed naturally.

But Why Does It Persist for Some Learners?

For students with Dyslexia or other learning differences, this confusion can last longer. Recent studies (linked below) show that learners with language-based difficulties also have more difficulty suppressing the invariance. 

This is because they need more explicit teaching of letter-sound connections, and often, if there are underlying working memory or slow automatised rapid naming issues, then these differences can make learning the connections and recalling them harder. 

It’s really important to remember that it’s not laziness; there is a distinct difference in neurological processing. 

Best Science of Learning Ways to Support It

So the parents of my lovely student today wanted some tips on how to support this at home. I explained that it basically takes a lot of repetition! 

Here’s what actually helps (and what we use in structured literacy):

1. Explicit Teaching (Don’t Assume It Will “Click”)
Teach each letter clearly and directly:

• This is b → /b/

• This is d → /d/

Don’t teach them straight after each other. Do one, then later, do the other.

2. Anchor the Letters to Meaning
Give each letter a strong “hook”:

• b → “bat before ball”

• d → “c into d”

👉 This helps the brain lock in direction.

3. Multi-Sensory Practice
Get the body involved:

• Air writing

• Tracing while saying the sound

• Writing the letter over and over whilst simultaneously saying the sound

• Writing the letter, saying the sound with eyes closed.

4. Contrast Practice
Query them together:

• What is the same?

• What is different?

👉 This builds discrimination, not confusion.

5. Link Reading and Writing
Don’t just recognise letters—write them:

• Say the sound

• Write the letter

• Use it in words

• Write those words

6. Provide Lots of Supported Practice
Repetition matters, but it must be:

• Accurate

• Supported

• Purposeful

Final Thoughts

When children mix up b, d, p, and q, they are not doing something wrong. They are showing us that their brain is doing something very clever, and very human. Our job is simply to help them retrain that system for reading.

Further Reading:
1. “Breaking the Symmetry” – Stanislas Dehaene and colleagues

2. “Selective Inhibition of Mirror Invariance for Letters” (2021)

3. “How Literacy Breaks Mirror Invariance” (Pegado et al.)