First of all, spells are something that C3 mages use.

We will be starting with a simple theory, out of all the spells that have been used. There are children's songs and rhymes that most of not all are based off of (I cannot find all songs to go with spells, feel free to send some you may think the spells are based off of that haven't been included)

I can't seem to find one of Touma's spell but Ruu mentioned to me that spell involved something like "Daddy Long Legs" which with some researched I discovered is actually a song by the name of "My Daddy Long Legs". Another one of his spells involved the song "Rock-A-Bye-Baby".

In the house of "Nobody Cares,"
Little "Miss No-one" lived with her prayers;
Never knowing love's tender hug,
She longed to find someone to love;
One day love came her way,
And the whole world heard her say:
"My Daddy Long Legs,
You've made my life worthwhile;
My Daddy Long Legs,
You've taught me how to smile,
When I saw your shadow on the wall,
The one that the sunbeams drew;
My Daddy Long Legs,
I fell in love with you."

Who turned all the grey clouds to blue,
My Daddy Long Legs, no one but you;
Who dried all the tears in my eyes,
Who made the sun shine in the skies?
Ev'ry prayer that I say;
I'll forever end this way:
"My Daddy Long Legs,
You've made my life worthwhile;
My Daddy Long Legs,
You've taught me how to smile,
When I saw your shadow on the wall,
The one that the sunbeams drew;
My Daddy Long Legs,
I fell in love with you."

Rock-a-bye, baby
On the treetop
When the wind blows
The cradle will rock
If the bough breaks
The cradle will fall
But mama will catch you
Cradle and all

Baby is drowsing
Cozy and fair
Mother sits near
In her rocking chair
Forward and back
The cradle she swings
And though baby sleeps
He hears what she sings

Hush a bye baby
Up in the sky
On a soft cloud
it's easy to fly
Angels keep watch
Over as you sleep
So hush a bye baby
Don't make a peep

Rock a bye baby
Do not you fear
Never mind baby
Mother is near
Wee little fingers
Eyes are shut tight
Now sound asleep
Until morning light

Next up is Yumikage's spell. The only song I could find that goes along with some of his spell is "Who Killed Cock Robin", but I feel like his spell does or should have more songs included.

Who killed Cock Robin?
I, said the Sparrow,
with my bow and arrow,
I killed Cock Robin.

Who saw him die?
I, said the Fly,
with my little eye,
I saw him die.

Who caught his blood?
I, said the Fish,
with my little dish,
I caught his blood.

Who'll make the shroud?
I, said the Beetle,
with my thread and needle,
I'll make the shroud.

Who'll dig his grave?
I, said the Owl,
with my pick and shovel,
I'll dig his grave.

Who'll be the parson?
I, said the Rook,
with my little book,
I'll be the parson.

Who'll be the clerk?
I, said the Lark,
if it's not in the dark,
I'll be the clerk.

Who'll carry the link?
I, said the Linnet,
I'll fetch it in a minute,
I'll carry the link.

Who'll be chief mourner?
I, said the Dove,
I mourn for my love,
I'll be chief mourner.

Who'll carry the coffin?
I, said the Kite,
if it's not through the night,
I'll carry the coffin.

Who'll bear the pall?
We, said the Wren,
both the cock and the hen,
We'll bear the pall.

Who'll sing a psalm?
I, said the Thrush,
as she sat on a bush,
I'll sing a psalm.

Who'll toll the bell?
I said the bull,
because I can pull,
I'll toll the bell.

All the birds of the air
fell a-sighing and a-sobbing,
when they heard the bell toll
for poor Cock Robin.

The next spell is Shuuhei, his is based from "Sing a Song of Sixpence". His song, unlike the others is found throughout his spell, it continuously returns to the same part of the song.

Sing a song of sixpence,
A pocket full of rye,
Four and twenty blackbirds
Baked in a pie.

When the pie was opened
The birds began to sing—
Wasn't that a dainty dish
To set before the king?

The king was in the counting-house
Counting out his money,
The queen was in the parlor
Eating bread and honey,

The maid was in the garden
Hanging out the clothes.
Along came a blackbird
And snipped off her nose.

Then there's Jun's spell which I have yet to find anything with my research.

Tsurugi's spell where he says to pat his head and call him good seems to make no sense, it was not included because considering Tsurugi's broken mind I figured that his spells would also be broken. A broken man spouting broken nonsense that gives him power.

Mikuni's lead activation also includes part of a Song, sense he was songs part of C3 it makes sense for him to be included. His lead activated as follows: "As you like it, my fair lady!", which for those of you don't know can be found through the kids song "London Bridges Falling Down"

Anyways Ruu came up with the conclusion in which the longer part of the song from the spell is, the stronger they are. Tsurugi was very strong and they had no song. Touma's was also strong and he only had one song per spell and the lyrics weren't the same as the song.

Shuhei took many part of a song and it only made him able to see anything.

Thank you for reading this theory on where C3's spell ideas come from. It is just a theory so it may not be true and if it is true it may not be for all spells.