
Most of us have probably learnt the [Knuckle mnemonic](https://en.wikipedia.org/wiki/Knuckle_mnemonic), the following poem, or some [variant](https://en.wikipedia.org/wiki/Thirty_Days_Hath_September) of it at some point to remember the number of days in each month:

```plaintext
Thirty days has September,
April, June, and November,
All the rest have thirty-one,
Save February at twenty-eight,
But leap year, coming once in four,
February then has one day more.
```

It's cute but mostly unnecessary. It doesn't take a whole lot to remember that February has 28 days in a common year, and 29 days in a leap year, which happens every four years.

So the only valuable info in that poem is this section: **September, April, June, and November** which still requires some effort to recall. To make it stick, I came up with my own mnemonic back in middle school: **SNAp June**, and I've relied on this successfully for years. 

In numeric format, these months correspond to **4 6 9/11**.

All the other months consist of 31 days.


An easy mnemonic for the number of days in each month

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I know, this is a completely worthless thing to memorize. Even [NASA](https://www.jpl.nasa.gov/edu/news/2016/3/16/how-many-decimals-of-pi-do-we-really-need/) uses a mere 15 digits of Pi for their highest precision calculations, and according to them, just 38 digits of Pi are sufficient to calculate the circumference of the entire observable universe to an accuracy equal to the diameter of a hydrogen atom. But hear me out, this is a trick that's worth learning if you want to impress your friends, especially considering how easy it is to learn!

The technique is a standard method used by professional memory champions. So if you learn it for this trick, you can use it for many other use cases, like memorizing phone numbers and even the order of a shuffled deck of cards. It's called "**major system**". This allows you to map each digit into a distinct sound and construct mnemonics that are easy to remember. See the table below:

| Numeral | Associated letters |
| ------- | ------------------ |
| 1       | t, d               |
| 2       | n                  |
| 3       | m                  |
| 4       | r                  |
| 5       | l                  |
| 6       | j, sh, ch, soft g  |
| 7       | k, hard g, hard c  |
| 8       | f, v, ph           |
| 9       | p, b               |
| 0       | z, s, soft c       |

Let's look at an example. Say you want to remember the number 42. Since it maps to [r, n], you can choose any word that has these consonant sounds like _rain_, _run_, or _ruin_. Similarly, 620 could be _jeans_, _Jonas_, or _chains_. An important property of this system is that although a number can be converted into many words, a word can be translated only into a single number.

Now let's get to the good stuff. Try to remember the following sentence:

> **My turtle Pancho will, my love, pick up my new mover, Ginger.**

Done? Congratulations, you now know the first **twenty-four** digits of Pi! Here's how it works. If you map the consonant sounds in that sentence back to the digits from the table, you get the following:

```plaintext
My turtle Pancho will, my love, pick up my new mover, Ginger.
3  1415   926    5     3  58    97   9  3  2   384    6264
```

Let's go a bit further. Add these sentences to the previous one and you've got the first **sixty** digits.

```plaintext
My movie monkey plays in a favorite bucket.
3  38    327    950   2    8841     971

Ship my puppy Michael to Sullivan’s backrubber.
69   3  99    375     1  05820      97494
```

Finally, let's crank it up to a **hundred** digits.

```plaintext
A really open music video cheers Jenny F. Jones.
  45     92   307   81    640    62    8  620

Have a baby fish knife so Marvin will marinate the goosechick.
8      99   86   28    0  3482   5    3421     1   7067
```

Here are all the sentences written together:

> **My turtle Pancho will, my love, pick up my new mover, Ginger. My movie monkey plays in a favorite bucket. Ship my puppy Michael to Sullivan’s backrubber. A really open music video cheers Jenny F. Jones. Have a baby fish knife so Marvin will marinate the goosechick.**

## Further reading

By the way, I learned this trick from [Prof. Arthur Benjamin's](https://www.youtube.com/watch?v=e4PTvXtz4GM) excellent book [Secrets of Mental Math](https://www.goodreads.com/book/show/83585.Secrets_of_Mental_Math). This book is full of mental math techniques and I highly recommend it. Here are some other links of interest:

- [Mnemonic major system - Wikipedia](https://en.wikipedia.org/wiki/Mnemonic_major_system)
- [major-system.info](https://major-system.info/en/) - Major system mnemonic database and converter.
- [Piphilology - Wikipedia](https://en.wikipedia.org/wiki/Piphilology)
