The framework deployed in this program is Bauer & Nation's (1993) model of the most useful word affixes, divided into basewords and six levels of affixation according to the following features of learnability: their meaning (whether affixes changes the meaning or part of speech of the base word, like un~), their grammar (whether affix changes the part of speech, i.e. are derivational) or do not (are inflectional)); their frequency (number of occurrences in a corpus), their regularity (degree of change they impose on the base word), their productivity (whether they can be applied to a wide range of base words, including novel), and their predictability (whether they have standard meanings, as '~able' and '~less' do but '~ate' does not).

Minor changes in classifications have been made to render these lists computational (see the paper for differences) including the addition of ~or as a version of ~er when added to verbs in Level 3 and ~est as an inflection similar to ~er when added to adjectives in Level 2).

Level 1 Base words
Level 2 Base words + inflections ('lemmas')
~s (on noun or verb), ~ed/~ing (on verb), ~er (er2)/~est (on adjective), ~th (on number), and ~en (en2) on irregular verb
~er2 and ~en2 are named to separate them from verb+er in Level 3 and noun+en in Level 5

All the rest involve derivations (change in meaning and/or part of speech)

Level 3 Frequent and regular affixes with minimal change to the base word in speech or writing
~able/ible, ~er/~or (on verb), ~ish, ~less, ~ly, ~ness, ~th, ~y,
non~, un~
Level 4 Frequent orthographically regular affixes which often impose pronunciation change (admIre => admirAtion)
~al (autumnal), ~ation (admiration), ~ess (fortress), ~ful (plentiful), ~ism (dogmatism), ~ist (semanticist), ~ity (solemmnity), ~ize (serialize), ~ment (armament), ~ous (fibrous)
in~, im~
Level 5 Less frequent but regular affixes
~age (leakage), ~al (arrival), ~ally (idiotically), ~an (American), ~ance (clearance), ~ant (consultant), ~ary (revolutionary), ~atory (confirmatory), ~dom (kingdom; officialdom), ~eer (black marketeer), ~en (wooden), ~en (widen), ~ence (emergence), ~ent (absorbent), ~ery (bakery; trickery), ~ese (Japanese; officialese), ~esque (picturesque), ~ette (usherette; roomette), ~hood (childhood), ~i (Israeli), ~ian (phonetician; Johnsonian), ~ite (Paisleyite; also chemical meaning), ~let (coverlet), ~ling (duckling), ~ly (leisurely), ~most (topmost), ~ory (contradictory), ~ship (studentship), ~ward (homeward), ~ways (crossways), ~wise (endwise; discussion~wise),
anti~ (anti~inflation), ante~ (anteroom), arch~ (archbishop), bi~ (biplane), circum~ (circumnavigate), counter~ (counter~attack), en~ (encage; enslave), ex~ (ex~president), fore~ (forename), hyper~ (hyperactive), inter~ (inter~African, interweave), mid~ (mid~week), mis~ (misfit), neo~ (neo~colonialism), post~ (post~date), pro~ (pro~British), semi~ (semi~automatic), sub~ (subclassify; subterranean), un~ (untie; unburden).
Level 6 Frequent but irregular affixes (often with significant change to base word)
~able (inscrutable), ~ee (lessee), ~ic (spastic), ~ify (mollify), ~ion (superstition), ~ist (solipsist), ~ition (transition), ~ive (restive), ~th (breadth), ~y (calumny),
pre~, re~
Level 7 Classical affixes
~ar (circular), ~ate (electorate), ~et (packet, casket), ~some (troublesome), ~ure (departure, exposure)
ab~ (abnormal), ad~ (admixture), com~ (commiserate), de~ (demist), dis~ (disintegrate), ex~ (out ~ external), in~(in ~ internal), ob~ (obsequious), per~ (perspective), pro~ (in front of ~ procede), trans~ (transmogrification)