Letter Pairs

[Türkçe]

This list was born from a random thought I had during a long 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 (by Cem). A (confirm) B (cancel) buttons on game consoles (by Victor). Alice and Bob in cryptography.
A	D	In computer games, move left and right. Analog to digital converter (“A-to-D”) (by Joaquin).
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”) (by Mert).
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. Business-to-business (“B2B”).
B	C	Business-to-customer (“B2C”).
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	R	Rhythm and blues (R&B).
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 (by Joaquin).
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 (by Aaron). (Non-sound recording) copyright symbol (C) and sound recording copyright symbol (P).	CP violation.
C	S	Strange and charm quarks (second generation) (by Josephine).
D	D	Dungeons & Dragons (“D&D”). The denial and deception (“D&D”) framework in military intelligence.	Why do I allow Dungeons & Dragons but not D&G for Dolce and Gabbana?
D	E	Difficulty and execution scores in gymnastics.
D	K	Kill/Death ratio in computer games.
D	L	Dextrorotary and levorotary compounds in chemistry (by Azra).
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) (by Josephine).
E	F	Empty and full in fuel indicators in cars.
E	P	Proton and electron. Price–earnings ratio in finance (“P/E ratio”) (by Joaquin).	Perhaps add neutron (N).
E	S	Episode and season in TV series (e.g. “S3 E4”).
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	K	Coordinates for the vertex of a parabola (by Kerem).
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 (by Arnav).
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).	S&L: Also known as a thrift institution.
L	W	Length and width for 2D objects (by Joaquin). 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 (by Mert).
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) (by Alex Fowler).
O	$\Omega$	Time complexity functions.	what about $\theta$?
P	P	pp junction.
P	Q	“Mind your Ps and Qs” (by Eleni). Price and quantity in economics.
P	R	PR interval in cardiology.
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 (by Ravi). QT interval in cardiology.	“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-R variability in cardiology (by Gamze Kılıçoğlu).
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 (by Josephine).
R	V	Recto (“right” or “front” side) and verso (“left” or “back” side) in printing (by Ned).	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. ST depression in cardiology (by Gamze Kılıçoğlu).
S	W	In computer games, move up and down.
W	Z	Avian sex chromosomes (male) (by Raymond). 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) (by Raymond).
$\Alpha$	$\Omega$	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) (by Joaquin).