Letter Pairs

This list was born from a random thought I had during an international flight. I realized that there are many scenarios where two letters are used to denote two opposite things, like the X and the O in tic-tac-toe. The list was initially supposed to only contain letters that represent opposite things, but I decided to include any pair of letters that frequently go together. I will be adding more pairs as I find them. Feel free to let me know if you have any suggestions.

Some invalid examples:

Pyjamas are also called PJs but the P and J there would be part of the same thing, not two different things. The letters should symbolize different things.
I and B for investment banking: two separate words, but they are still part of the same thing.
A and F because they are the two extremes of the letter grade system: there are other letters in the system, too. There is no context where we forget about the other letter grades and only focus on A and F. Note that P and F for pass and fail is valid because there aren’t any other letters.

Letter 1	Letter 2	Context	Comments
A	B	Blood type. A/B testing in user experience research. A (confirm) B (cancel) buttons on game consoles. Alice and Bob in cryptography.
A	D	In computer games, move left and right. Analog to digital converter (“A-to-D”).
A	E	Accident and emergency department (A&E).	E.g. emergency room.
A	G	Goal and assist.
A	M	Mergers and acquisitions (M&A).
A	Q	Question and answer (“Q&A session”).
A	S	Side and angle in triangle congruence rules (SAS, SSS, ASA). Soprano and alto in the SATB human voice type system.
A	T	DNA diagrams (adenine and thymine).
A	U	RNA diagrams (adenine and uracil).
A	Z	Latin alphabet first and last letters.	concrete example: “sort alphabetically” symbol in Google Docs.
B	B	Bed and breakfast.
B	E	Electric ($\bm{E}$) and magnetic ($\bm{B}$) fields.
B	F	Rubik’s cube moves, Singmaster notation (front and back).
B	P	Price-to-book ratio in finance.
B	T	Top and bottom quarks (third generation). Tenor and base in the SATB human voice type system.
B	U	U-B color index in astronomy.	Wikipedia says also V-R and R-I?
B	V	B-V color index in astronomy.	Wikipedia says also V-R and R-I?
B	W	Chess notation (black and white). Black and white color scheme.
C	C	Command and control (C&C) in cybersecurity.
C	F	Fahrenheit and Celsius.	What about Kelvin?
C	G	DNA diagrams (guanine and cytosine).
C	H	Hot and cold in faucets.	But it’s usually a snowflake and the Sun.
C	M	Message and ciphertext in cryptography.
C	P	C-symmetry (charge conjugation symmetry) and P-symmetry (parity symmetry) in particle physics.	CP violation.
C	S	Strange and charm quarks (second generation).
D	D	Dungeons & Dragons (“D&D”).	Why do I allow this but not D&G for Dolce and Gabbana?
D	K	Kill/Death ratio in computer games.
D	L	Dextrorotary and levorotary compounds in chemistry.
D	R	Automatic cars (drive and reverse). Democrat and Republican in the US. Research and Development.
D	U	Rubik’s cube moves, Singmaster notation (up and down). Up and down quarks (first generation).
E	P	Proton and electron.	Perhaps add neutron (N).
E	V	Vertex and edge sets of a graph ($G = (V, E)$).
E	W	Compass (East and West).
F	M	Sex in ID cards (male and female).
F	P	Pass and fail in an academic transcript. Piano and forte in music notation.
F	T	True and false. Frequency ($f$) and period ($T$).
F	$\lambda$	Frequency ($f$) and wavelength ($\lambda$).
G	T	Gin and tonic.
H	L	HL triangle congruence rule (hypotenuse and leg).	Also known as the RHS (right-angle-hypotenuse-side) rule.
H	T	Coin flip (head and tail).	Might remove later.
I	O	Input and output. On and off.
I	R	Real ($\Re(z)$) and imaginary ($\Im(z)$) parts of $z \in \mathbb{C}$.
I	U	You and I.
K/T	U/V	Kinetic (K or T) and potential (U or V) energy.
L	P	Profit and loss in finance.	“P&L statement”.
L	R	Rubik’s cube moves, Singmaster notation (left and right). Printing L (left) or R (right) in X-ray scan results to identify the leg/arm. Right and left in shoes.
L	S	Savings and loan association (S&L). Lumbar (L1 to L5?) and sacral (S1 to S5?) vertebrae.	S&L: Also known as a thrift institution.
L	W	Length and width for 2D objects. Win and loss (or winner and loser).
M	M	M&M, the chocolate confectionery.	Was initially a joke but why not.
M	W	Spinning a Wilson tennis racket to simulate a coin flip (“M or W?”). Man and woman.
N	N	nn junction.
N	P	pn junction in electronics.
N	S	Compass, magnet (North and South). Signal to noise ratio (“S/N”).	“SNR” is more common than “S/N”.
N	Y	Questionnaires/Exams (yes and no).
O	T	Medieval T and O maps.
O	U	Over and under in betting.
O	X	Tic-tac-toe. Hugs (X) and kisses (O).
O	$\Omega$	Time complexity functions.	what about $\theta$?
P	P	pp junction.
P	Q	“Mind your Ps and Qs”. Price and quantity in economics.
P	S	S and P waves in seismology. Salt and pepper. S-type and P-type planet orbits in a binary star system.	“Planets that orbit just one star in a binary system are said to have “S-type” orbits, whereas those that orbit around both stars have “P-type” or “circumbinary” orbits.” – Wikipedia
P	T	Beach ball diagrams in geology.
P	V	Prover and verifier in cryptography.
P	X	Position (X) and momentum (P).	E.g. Heisenberg uncertainty principle.
Q	T	To tap (T) or untap (Q) a permanent in Magic: The Gathering.	“Traditionally, the symbols are denoted with curved arrows, but in written notation in the glossary of comprehensive rules, they use these alphanumeric symbols as well.” – Ravi
R	R	Rest and relaxation.
R	S	Set (S) and reset (R) for SR latch. Enantiomer orientations in chemistry. $r$ (rotations) and $s$ (reflections) in $D_{2n}$ (dihedral group). s-process (slow), r-process (rapid) neutron capture in the centers of stars.
R	V	Recto (“right” or “front” side) and verso (“left” or “back” side) in printing.	Especially when talking about a folium.
R	W	Read/Write permissions in computing.
R	$\theta$	Polar coordinates.
S	T	Sales and trading division in an investment bank.
S	W	In computer games, move up and down.
W	Z	Avian sex chromosomes (male). W and Z bosons.
X	X	Sex chromosomes in a female human.
X	Y	2D Cartesian system. Sex chromosomes in a male human.
Y	$\Delta$	Y-$\Delta$ transformations in discrete geometry.
Z	Z	Avian sex chromosomes (female).
$\Alpha$	$\Omega$	Christian symbol (Alpha and Omega).
$\delta$	$\epsilon$	$\epsilon$-$\delta$ proofs in mathematical analysis.
$\varepsilon$	$\sigma$	Stress–strain curves.	Is $\tau$ for stress and $\gamma$ for strain also valid?
$P$	$\Chi$	Christian symbol (Chi Rho).
$\Phi$	$\Psi$	Torsion angles in molecular polypeptide chains (e.g. Ramachandran plot).