~ 14 ~ Fourteen ~

'our diatonic leading tone 7th up an octave.'

In a nutshell. 'Take it up an octave and leave it out.' There's really no such critter ever mentioned in the Essentials performance / theory dynamic or anywhere else for that matter. Don't ever recall a pitch within real music that was analyzed or referred to as any kind of '14th.' We can half it to Seven and have a real, super robust monster of a working tone though. There's a ton of coolness on Seven almost all of which is essential ... in theory that is :)

So why 14? Simply that in by numbers, Fourteen gets us up another octave above our starting pitch. Examine the pitches and their corresponding numbers. Example 1.
scale degrees
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
two octave C major scale
C
D
E
F
G
A
B
C
D
E
F
G
A
B
C

Review and forward. The theory is getting pretty thin up here in this atmosphere Jacmuse ... :) Better save it with a good quote !
"Silence is a great help to a seeker after truth."
Footnotes:

(1) Isacoff, Stuart. Temperament ... The Idea That Solved Music's Greatest Riddle, p. 40-42. USA Alfred A. Knopf, New York. 2001
(1)Duffin, Ross W. How Equal Temperament Ruined Harmony, p.32. USA W.W.Norton and Company, NY, New York. 2007.
(2)Aebersold, Jamey and Slone, Ken. The Charlie Parker Omnibook. New York: Atlantic Music Corp., 1978.

Russell, George. The Lydian Chromatic Concept Of Tonal Organization. USA Concept Publishing Company, Cambridge, Mass. 1982