The English alphabet (above) on the nursery wall
Letters on the schoolroom wall
An alphabet was a set of letters used for the writing of languages. Words written in order of their first letters were said to be in alphabetical order. (TNG: "The Mind's Eye") They were also used in mathematical equations to stand in for unknown numbers.
The Greeks on Earth used their own Greek alphabet. (VOY: "The Omega Directive")
In 2152, a crewmember asked Captain Archer for the name of the first Vulcan ambassador to Earth for her crossword puzzle, consisting of six letters, the last one being an "R". (ENT: "The Catwalk")
The musical notes on the Preservers' obelisk on Amerind, when interpreted into tones, corresponded roughly to an alphabet. (TOS: "The Paradise Syndrome")
The alphabet of the English language was displayed on the wall of the nursery on the USS Enterprise-D. (TNG: "The Child")
After a Federation starship with a particularly famous name and registry number was decommissioned or destroyed, subsequent ships could have letters of the alphabet attached to the pre-existing registry. (TNG: "Relics"). Among the most notable examples was the USS Enterprise. So notable was the exploits of the ship and crew that after its destruction in 2285 Starfleet gave subsequent starships named Enterprise the NCC-1701 registry with letters appended, starting with USS Enterprise-A. (Star Trek IV: The Voyage Home)
After hearing Jean-Luc Picard call out to Q in 2364, Worf asked Tasha Yar what a Q was. She replied that it was a letter of the alphabet, at least to her knowledge. (TNG: "All Good Things...")
In 2367, in a game Geordi La Forge played with the computer of the Onizuka, it asked La Forge to list the resonances of the subquantum states associated with transitional relativity in twenty seconds, a task which he initially considered easy until the computer clarified that it was to be in alphabetical order. He made it up to four before time ran out, though he knew the final answer. (TNG: "The Mind's Eye")
An artifact, stolen by the crew of Baran's mercenary ship from Calder II in 2370, had an alphabet and symbology much closer to early Vulcan language than the Romulan language according to Jean-Luc Picard. (TNG: "Gambit, Part II")
After Captain Picard had initiated the auto-destruct of the USS Enterprise-E in 2373, Beverly Crusher asked him if Starfleet would build a new Enterprise. Picard replied optimistically as there were "plenty of letters left in the alphabet". (Star Trek: First Contact)