| Letter-ending abbr. | 19 |
| Letter-drop site: abbr. | 23 |
| Letter-closing words | 20 |
| Letter-closing word | 19 |
| Letter-box inscription | 22 |
| Letter-bottom letters | 21 |
| Letter-bottom abbr. | 19 |
| Letter, of sorts | 16 |
| Letter, in Soho | 15 |
| Letter, in Madrid | 17 |
| Letter's sign | 17 |
| Letter's rest period? | 25 |
| Letter's paper | 18 |
| Letter's number | 19 |
| Letter's need | 17 |
| Letter's must-have | 22 |
| Letter's first word | 23 |
| Letter's end? | 17 |
| Letter's cousin | 19 |
| Letter's attachment (Abbr.) | 31 |
| Letter's antecedent | 23 |
| Letter you can read without the light on | 40 |
| Letter Y sporter | 16 |
| Letter X? | 9 |
| Letter written as a straight vertical line | 42 |
| Letter writing: Abbr. | 21 |
| Letter writing, some would say | 30 |
| Letter writing, some say | 24 |
| Letter writing, it's said | 29 |
| Letter writing, as some would say | 33 |
| Letter writing and blacksmithing | 32 |
| Letter writer? | 14 |
| Letter writer, formally | 23 |
| Letter writer's signoff | 27 |
| Letter writer's sign-off | 28 |
| Letter writer's embellishment | 33 |
| Letter writer's aid | 23 |
| Letter writer's afterthoughts | 33 |
| Letter writer's 52? | 23 |
| Letter writer | 13 |
| Letter with the largest section in a dictionary | 47 |
| Letter with jay or tee | 22 |
| Letter with a right angle | 25 |
| Letter with a limited amount of space | 37 |
| Letter with a double twist | 26 |
| Letter with a double curve | 26 |
| Letter with a 90-degree bend | 28 |
| Letter winner | 13 |
| Letter wearers, e.g. | 20 |
| Letter wearer: Abbr. | 20 |
| Letter voiced by a Greek giant? | 31 |
| Letter vendor? | 14 |
| Letter used with theta and phi to represent spherical coordinates | 65 |
| Letter used to represent resistance | 35 |
| Letter used to represent Lagrange multipliers | 45 |
| Letter used as a density symbol | 31 |
| Letter upon which this puzzle's theme is built | 50 |
| Letter two before iota | 22 |
| Letter turner | 13 |
| Letter trio | 11 |
| Letter train | 12 |
| Letter toppers | 14 |
| Letter to the editor that might be received immediately | 55 |
| Letter to Santa, essentially | 28 |
| Letter to Santa, e.g. | 21 |
| Letter to Santa contents | 24 |
| Letter to Odin? | 15 |
| Letter to a Viking? | 19 |
| Letter to a Viking | 18 |
| Letter thought to resemble a skull | 34 |
| Letter that, as it appears in the middle of this grid, can precede the first words of the starred entries | 105 |
| Letter that's opened with a double-click | 44 |
| Letter that's not really at the end of the Greek alphabet | 61 |
| Letter that's also a name | 29 |
| 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 |