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. Note that P and F for pass and fail is valid because there aren’t any other letters.
| Letter 1 | Letter 2 | Context | Comments |
|---|---|---|---|
| X | O | tic-tac-toe | |
| N | S | compass, magnet (North and South) | |
| E | W | compass (East and West) | |
| $\Alpha$ | $\Omega$ | Christian symbol (Alpha and Omega) | |
| $\Chi$ | $P$ | Christian symbol (Chi Rho) | |
| I | O | input/output or on/off | |
| Y | N | questionnaires/exams (yes and no) | |
| M | W | Turning a tennis racket to simulate coin flip (“M or W?”) | |
| A | B | blood type, A/B testing in user experience research | |
| B | W | chess notation (black and white) | |
| H | T | coin flip (head and tail) | might remove |
| A | T | DNA diagrams (adenine and thymine) | |
| G | C | DNA diagrams (guanine and cytosine) | |
| A | U | RNA diagrams (adenine and uracil) | |
| X | Y | 2D Cartesian system, sex chromosomes | |
| W | L | win and loss (or winner and loser) | |
| F | B | Rubik’s cube moves, Singmaster notation (front and back) | |
| U | D | Rubik’s cube moves, Singmaster notation (up and down) | |
| L | R | Rubik’s cube moves, Singmaster notation (left and right) | |
| R | I | Real ($\Re(z)$) and imaginary ($\Im(z)$) parts of $z \in \mathbb{C}$ | |
| T | P | Beachball diagrams in geology | |
| P | E | Proton and electron | perhaps add neutron (N) |
| D | R | Automatic cars (drive and reverse), Democrat and Republican in the US, Research and Development | |
| A | D | In computer games, move left and right | |
| W | S | In computer games, move up and down | |
| M | F | Sex in ID cards (male and female) | |
| T | O | Medieval T and O maps | |
| S | P | S and P waves in seismology | |
| E | B | Electric ($\bm{E}$) and magnetic ($\bm{B}$) fields | |
| H | C | Hot and cold in faucets | but it’s usually a snowflake and the Sun |
| r | $\theta$ | Polar coordinates | |
| $\epsilon$ | $\delta$ | $\epsilon$-$\delta$ proofs in mathematical analysis | |
| M | M | M&M, the chocolate confectionery | was initially a joke but why not |
| U | I | You and I | |
| Y | $\Delta$ | Y-$\Delta$ transformations in discrete geometry | |
| P | F | Pass and fail in an academic transcript | |
| G | T | Gin and tonic | |
| K/T | U/V | Kinetic (K or T) and potential (U or V) energy | |
| X | P | Position and momentum | e.g. Heisenberg uncertainty principle |
| A | Z | Latin alphabet first and last letters. | concrete example: “sort alphabetically” symbol in Google Docs |
| M | A | Mergers and acquisitions | |
| S | T | Sales and trading division in an investment bank | |
| K | D | Kill/Death ratio in computer games | |
| F | C | Fahrenheit and Celsius | what about Kelvin? |
| R | R | Rest and relaxation | |
| Q | A | Question and answer (“Q&A session”) | |
| P | N | p-n junction in electronics | |
| S | R | Set (S) and reset (R) for SR latch | |
| D | D | Dungeons & Dragons (“D&D”) | |
| Z | W | Avian sex chromosomes | |
| T | Q | 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 |