//------------------------------//
// Sparity 
// Story: Shipping Labels 
// by Eruantalon 
//------------------------------//

"How's your day been, Rarity?"

Rarity smiled at the young dragon.  "Oh my!  There was a most adorable young mare who came on the train from Canterlot all to order a dress from my boutique!"

"Wow!"  Spike's eyes went wide.  "What for?"

"She's going to be graduating Princess Celestia's School next week, and of course she wants to look her best...  Hand me that golden thread, if you would?  No, the thinner one."

Spike carefully got the skein of thinnest (and thus finest-quality) golden thread off the rack and handed it over to Rarity.  "I'd say that's an excellent day, Rarity!"

"Oh, absolutely!"  She flashed Spike a broad smile and then bent down to thread a needle in her magic.  "So how was your day?"

"Twilight and I've been studying how magic moves through space when you flip it over."

"Oh?"  She looked up, her needle poised over some golden cloth.

"Like..."  He looked around the shop, and his eyes lit on a plain white dress without any decorations.  "Like that dress over there.  Its left and right sides are just the same, so when you look at it in a mirror, it looks just the same as the real dress."

"All right," Rarity said with a nod, as she started to stitch.

"And that's called 'parity.'  But if you put a daisy or something on one side of the dress, it wouldn't have parity anymore."

Rarity nodded again, drew the needle out of the cloth, and stood frowning.  "So how does this relate to magic?"

"Well, Twilight and I've been studying the Poynting vector of... well, how the energy moves from your horn through space.  And we've found that it doesn't have parity."

Rarity raised her eyebrows.  "The beam of magic looks the same on left and right."

"Well..."  Spike dashed over to a small board Rarity kept to sketch designs for her less imaginative and more demanding customers.  "So here's your horn, and here's the needle..."  He drew quick sketches.  "And here's the magic, and this arrow represents the magic flux through here.  That's the Poynting vector, and we call it 'S'."  He shrugged.  "Tradition.  So if the Poynting vector had parity, the S-parity would look like this..."