| Letter that sounds like a question | 34 |
| Letter that sometimes bounces | 29 |
| Letter that some feel should have its own day, rather than pi having Pi Day | 75 |
| Letter that rhymes with its two successors | 42 |
| Letter that rhymes with its preceding and following letter | 58 |
| Letter that rhymes with a colour | 32 |
| Letter that resembles the "y" of Disney's signature | 65 |
| Letter that requires no postage | 31 |
| Letter that requires clicking, not licking | 42 |
| Letter that originally represented an ox head | 45 |
| Letter that might include an emoticon | 37 |
| Letter that might go through a filter | 37 |
| Letter that looks the same upside down | 38 |
| Letter that looks like a pitchfork | 34 |
| Letter that looks like a horseshoe | 34 |
| Letter that forms a right angle | 31 |
| Letter that follows pi | 22 |
| Letter that follows epsilon | 27 |
| Letter that doesn't need a stamp | 36 |
| Letter that comes before upsilon | 32 |
| Letter that can't form words? | 33 |
| Letter that begins and ends this puzzle's six longest answers | 65 |
| Letter that appears twice in the Schrödinger equation | 56 |
| Letter that appears at least once in every single answer in this puzzle | 71 |
| Letter that appears 28 times in this grid | 41 |
| Letter than rhymes with the ones just before and after it | 57 |
| Letter takers | 13 |
| Letter taker, for short | 23 |
| Letter taker | 12 |
| Letter switched to ess in this puzzle's theme | 49 |
| Letter successor, to a large degree | 35 |
| Letter strokes | 14 |
| Letter stroke | 13 |
| Letter string | 13 |
| Letter stick-on | 15 |
| Letter steak | 12 |
| Letter starter of yore | 22 |
| Letter starter | 14 |
| Letter start | 12 |
| Letter sound | 12 |
| Letter smuggled past prison censors | 35 |
| Letter similar to a German Eszett | 33 |
| Letter signoff, for short | 25 |
| Letter signoff | 14 |
| Letter sign-off word | 20 |
| Letter sign-off wd. | 19 |
| Letter sign-off | 15 |
| Letter shuffles | 15 |
| Letter shuffle | 14 |
| Letter shaped like an upside-down U | 35 |
| Letter shaped like a P | 22 |
| Letter shaped like a horseshoe | 30 |
| Letter series | 13 |
| Letter sequence in the air | 26 |
| Letter sequence | 15 |
| Letter sent through cyberspace | 30 |
| Letter seen twice in Philadelphia | 33 |
| Letter sealer | 13 |
| Letter salutation | 17 |
| Letter run | 10 |
| Letter routing abbr. | 20 |
| Letter resembling H | 19 |
| Letter resembling an ox's head | 34 |
| Letter resembling an inverted "V" | 43 |
| Letter resembling a trident | 27 |
| Letter reqmts. | 14 |
| Letter recipient | 16 |
| Letter R, in Naples | 19 |
| Letter projection | 17 |
| Letter producer | 15 |
| Letter prevalent in Mississippi? | 32 |
| Letter presets used in place of arrows in keyboard-based computer games | 71 |
| Letter preceding iota | 21 |
| Letter preceding bravo | 22 |
| Letter permitting a cleric to transfer | 38 |
| Letter patterns | 15 |
| Letter paper? | 13 |
| Letter pair in ''pummel'' | 41 |
| Letter pair | 11 |
| Letter out for a stroll? | 24 |
| Letter or river | 15 |
| Letter or ray | 13 |
| Letter opening? | 15 |
| Letter opening | 14 |
| Letter openers? | 15 |
| Letter openers | 14 |
| Letter opener? | 14 |
| Letter opener: Abbr. | 20 |
| Letter opener, sometimes | 24 |
| Letter opener, of a sort | 24 |
| Letter opener, maybe | 20 |
| Letter opener, at times | 23 |
| Letter opener's creation | 28 |
| Letter opener | 13 |
| Letter on the AutoSum button in Excel | 37 |
| Letter on Superman's chest | 30 |
| Letter on some Swiss watches | 28 |
| Letter on some keys | 19 |
| Letter on some euros | 20 |
| Letter on some college sweatshirts | 34 |