Mobile Guide to Flats

~ = symmetric relationships
→ = asymmetric ones
+ = words manipulated together
, = words manipulated separately
− = deletion

Flat Types

Acrostical enigma (AE)
A word combines with initial letters from each clue to create new words.
See guide.

A word or phrase is divided into two or more others by taking alternate letters in order.
schooled → shoe + cold;
lacerated → let + are + cad

An anagram or antigram, depending on personal opinion.
See guide.

An appropriate transposal.
is tempo, sirs ~ prestissimo

An opposite transposal.
the budgerigar ≠ great huge bird (it's a parakeet)

A word or phrase becomes another when its last letter is changed (or "switched") and the remaining letters are reversed ("turned back"). See also Switchback.
autumn → mutual; dragnet → en garde

Baltimore deletion
Each letter in turn is removed to form a new word.
peat → eat, pat, pet, pea

Baltimore transdeletion
A word or entry phrase is turned into a series of others by removing each letter in turn and rearranging the rest.
baker → rake, kerb, bare, bark, beak

A word or phrase becomes another when its first letter is removed.
usable → sable; aspirate → spirate → pirate → irate → rate → ate

Brookline letter-change
A word or phrase changes each one of its letters in turn to make others.
rice → nice, race, rile, rich

Change of heart
A change of heart is similar to a Double-Cross in that two words or phrases switch a letter or string of letters, but the things being switched aren't necessarily located at the ends of words.
share + colt → sole + chart

A word or phrase becomes another when one letter changes to another and moves to another location.
goatherd → gathered (2nd to 7th)

A word or phrase is broken into two or more shorter ones.
outreached → outre + ached

Combination padlock
A padlock, with the added feature that overlapping the two words produces a fourth word or phrase.
scar + Arab → scarab → scab

Two or more words or phrases share the same consonants in the same order, with any number of vowels (incl. Y).
acorn ~ crayon ~ ocarina; begin ~ beguine ~ begonia

A word or phrase becomes another when its last letter is removed.
stingy → sting

A word or phrase becomes another when an interior letter is removed.
simile → smile

Double Acrostical Enigma (AE)
An AE with two solutions.

Two words or phrases are each divided into two pieces; then their second pieces are switched to form two others.
maids + rapture ~ mature + rapids
[ma/ids, rap/ture]

Two words are joined; then one word is dropped out to form another from the leftover letters.
reamer + itch → reach + merit
[rea(mer, it)ch]

A word or phrase is clued indirectly through wordplay. Generally, a riddle describes a thing and an enigma a word.
See guide.

Enigmatic rebus
Enigmatic is a red flag, warning you that the reading involves something other than simple manipulation of letters.
See guide.

False derivatives
A word or phrase becomes another when some grammatical change is inappropriately applied.
limb → limber (false comparative);
treat → retreat (false reiterative);
butter → buttress (false feminine, analogous to waiter → waitress)

"Flats we never finish reading" were added to the guide in 2004.
See guide

Head-to-tail shift
A word or phrase becomes another when its first letter is moved to its end.
emanate → manatee

Heart transplant
In a heart transplant, a letter or series of letters is taken from inside one word and transplanted to another.
clear + wild → car + willed

Two words or phrases with the same spelling are used with different pronunciations and meanings.
mustache ~ must ache

Two words or phrases sound like two other words that are antonyms.
knights, daze (sounds like nights, days)

Two words or phrases sound like two other words or phrases that form a familiar pair.
hart, sole (sounds like heart, soul)

Two or more unrelated words or phrases are pronounced the same but spelled differently.
hair ~ hare

Two words or phrases sound like two other words that are synonyms.
soul, loan (sounds like sole, lone)

Two or more shorter words or phrases are interlocked to form a longer one. (But not in a regular pattern like the alternade.)
fig + rebus → firebugs

Letter bank
A word or phrase (the "bank") is chosen that has no repeated letters. One or more longer words or phrases are formed, each using all the letters in the bank at least once and as many more times as needed.
lens → senselessness; manicures → Neiman Marcus, American sumac, marine insurance

Letter change
A specified letter is changed to make a new word or phrase.
3rd letter: pastry ~ pantry

Letter shift
A word or phrase becomes another when one letter is shifted to a new position.
Proust ~ sprout

A word or phrase is broken into two or more shorter parts, which overlap by exactly one letter.
libraries → Libra + Aries

Words or phrases are composed by pronouncing the letters of a base word individually or in combinations.
vacations → essay (SA) + ivy (IV) + eighty (AT) + envy (NV)

A word or phrase becomes another when two letters are interchanged.
converse ~ conserve

A transposal that is only vaguely appropriate or even entirely inappropriate.
See guide.

A reversed heteronym.
won ton ~ not now

The last two or more letters of a word or phrase are the same as the first two or more letters of another; joining the remaining letters forms another (the lock).
norther + thermal → normal

A sentence or phrase is spelled the same forward as backward.
Draw pupil's lip upward

Phrase Shift
In a phrase shift, an adage, well-known quotation, or cliche is remade by shifting a letter from one word to another.
The buck sops there; don't give UPS the hip

A hybrid of the rebus and alternade.
See guide.

A word or phrase is represented by letters, numbers, or symbols; their positions; and sometimes related letter-play, like alterations to the verse.
See guide.

Repeated letter change
A word or phrase becomes another when one letter is changed to another letter wherever it appears. The letter must appear at least twice and the change must be reversible.
crochet ~ prophet; skunk ~ stunt ~ sauna

Repeated letter or Word deletion
A word or phrase becomes another when one letter or word is removed wherever it occurs.
bassist − S → bait; prospered − P → rose-red; had it made − ad → hit me

A word or phrase becomes another when reversed.
desserts ~ stressed.

Reversed puzzles
A variation of a puzzle in which you have to reverse the result to read the final solution.
See guide.

Something is described enigmatically in verse. Generally, a riddle describes a thing and an enigma is a word.
See guide.

*Rochester transaddition
Each letter in a baseword is added back to the word and the result transposed.
nacre → canner, arcane, cancer, craner, careen

Sound change
One sound is changed in a word or phrase to make another.
legroom ~ legume

Sound shift
A word or phrase becomes another when its first sound is moved to its end.
ciao → ouch

A phrase or word becomes another when the initial consonant sounds in its component words (or stressed syllables) are swapped. Word boundaries do not need to be preserved.
key ring ~ reeking; trained seal ~ strained eel; rake over the coals ~ cake over the rolls

A hybrid of the suber and alternade or a reversed rebade.
See guide.

A reversed rebus.
See guide.

A word or phrase becomes another when its first letter is changed, or "switched", and the remaining letters are reversed, or "turned back". See also Backswitch.
hydra → tardy

Terminal deletion
A word or phrase is changed to another by removing its first and last letters.
foregone → Oregon

This flat type is always classified as a transdeletion.

A transposed charade. A word or phrase is broken up into two or more shorter parts. Each shorter part is transposed (separately) to make a word.
solu + tion → soul + into

A word or phrase becomes another when one letter is deleted and the others are transposed. There must be at least four parts and simple deletions are not used.
righteous − I → roughest - E → troughs − R → sought − U → ghost − G → shot − S → tho

A word or phrase becomes another when divided into two parts, which are interchanged.
rock-hard ~ hard rock; alloy ~ loyal

A word or phrase becomes another when its letters are rearranged. A flat is only classified as a transposal when it doesn't match a specific transposal type, such as metathesis, reversal or transpogram. [NOTE: A transposal is commonly called an anagram outside the NPL. In the NPL, an anagram is always an appropriate transposal.]
sleuth ~ hustle

Word deletion
A word or phrase is deleted from a longer one, leaving another word or phrase. In a multi-word deletion, two or more consecutive words are deleted. In a progressive word-deletion, three or more words are nested to form a longer one.
performance − man → perforce; organgrinder − gang rin → order; consecratory − Ra − sector → cony

Word substitution
A word or phrase is replaced by another in a longer one, producing a different word or phrase.
progress ~ congress [pro/con-gress];

Flat Modifiers

*Antonym puzzles
A puzzle variation using antonyms. For example, in an antonym substitution, replace a word with its antonym.
progress ~ congress (antonym substitution)

Bigram puzzles
Two-letter groups (bigrams) are the basic unit of these puzzles.
seraph ~ phrase (bigram reversal); we're aka Krewe (bigram palindrome)

Group puzzles
Each of the answer words or phrases is related to a member of a well-known group in the manner of a specified flat type.
sate, stew, shout, thorn ~ east, west, south, north (transposal group)

Phonetic puzzles
Puzzle variations in which sounds are the basic unit instead of letters.
quest → west (phonetic beheadment)

Progressive puzzles
A sequence of three or more of a given flat type.
padre, retrench, trenchant, anthem, hemlock, padlock (progressive padlock)

*Synonym puzzles
A puzzle variation using synonms. For example, in a synonym deletion, a synonym of the main base word has letters which are a subset of the letters in the main word. Remove those letters, and what remains, in that order, spells the second word. The synonym might be a synonym for a sense of the main word other than the one used in the flat. For a synonym substitution, replace a word with it's
against − anti → gas (synonym deletion)

Trigram puzzles
Three-letter groups (trigrams) are the basic unit of these puzzles.

Non-Guide Types

Alternate letters are taken from a word or phrase. Each set of letters is then transposed into another word.
lambskin → slim, bank

*Bridgewater transaddition
A generalization of the Rochester Transaddition. You begin with an n-letter BASE word and an m-letter BANK word (note: n and m do not need to be equal, as they are here). The other solwords, numbered ONE though M, are formed by transposing _all_ the letters in BASE plus each of the m letters from BANK in turn (so it's nice if BANK is an isogram). Another example: BASE = cast iron, BANK = atone, ONE = raincoats, TWO = tractions, THREE = consortia, FOUR = transonic, FIVE = creations.
cast iron + atone → raincoats, tractions, consortia, transonic, creations

A word or phrase is broken down into a series of shorter words and phrases by taking letters in order from the the original phrase - one from the front, then one from the back, then one from the front, and so on.
sapwort → star + pow

*Hutto transdeletion
A word or phrase has every possible pair of letters deleted, and each time the resulting set of letters is transposed to form another word. The cuewords in the following example indicate which pair of letters has to be deleted; the original word must also appear in the flat. This flat type generates a very large number of solution words: a base word of length 5 would require eleven; the example given has a seven letter base word, which results in twenty two words to be clued.
tirades → 12 dares, 13 ideas, 14 rides, 15 raise, 16 raids, 17 aired, 23 stead, 24 drest, 25 rates, 26 darts, 27 tread, 34 deist, 35 iteas, 36 staid, 37 Edita, 45 rites, 46 strid, 47 tried, 56 stair, 57 irate, 67 triad

*Lakeway changeover
A set of words such that any two of them form a changeover base, with every possible letter position included.
that, heat, hart, hats

*Letter Bankless
A letter bank where the minimal set of letters is not included in the base. This is most likely to be done because the minimal set will not transpose into a word.
green around the gills ~ Thousand Island dressing

*Lock & Drop
A lock and drop is like a padlock, except that in addition, the overlapping letters form a word.
lethal + halter → Hal + letter

*Multiple repeated-letter deletion
Like a repeated-letter deletion, except that all occurrences of any repeated letters are deleted.
statism → aim; catacomb → tomb

*Mutual replacement
Two letters replace each other each time they appear in a word.
sell → less; travail → trivial

*Name Change
A substring of one word is a proper name and is replaced by another proper name to form another word. The proper names are not individually clued.
Avalon → Avedon (Al → Ed); camel → cachet (Mel → Chet)

*Permuted consonantcy
In the Permuted Consonantcy, all permutations of the consonant letters appear once. For example, in a three-consonant flat, with all consonants different (which the following example is), the six squares might, in order, contain the following consonant sequences:
Consonants might also be repeated:
eclair, cairo IL, ulcer, lyric, oracle, relic

The base of a reduplication is such that the letters in the first half are repeated in the second half.
edit|ed it; Saint Theresa in the len|s. Ain't there Saint Helen

*Split Shift
In a split shift, two (or more) of the base words start and end with identical sequences of letters. Linking the dissimilar parts makes another word.
elegant + element → game

*Terminal letter change
The first and last letters of a word are both changed.
Spiderman ~ epidermal

*Terminal Rotation
A pair of words becomes another pair of words when all four terminal letters shift position in a manner analogous to rotating tires on a car: each front letter moves to the end, while the back letters move in front and switch words.
tend + bums → sent + dumb

A trans-cross is similar to a double cross, but the pieces of the word switch AB, CD, AC, BD.
seal + rely → sere + ally

An interlock in which one of the shorter pieces is tranposed. ?
an immediate action / a meditation / cinema

Most examples are taken from the Guide, where appropriate credits are given.