Superlatives in -errimus and -illimus

Two visible departures from regular -issimus are systematic enough to learn as classes. Adjectives whose masculine nominative singular ends in -er normally form their superlative in -errimus: ācer—ācerrimus “keen—keenest,” pulcher—pulcherrimus “beautiful—most beautiful.” Six common adjectives in -ilis form -illimus: facilis—facillimus “easy—easiest.”

These patterns affect superlative formation, not agreement. Once pulcherrim- or facillim- has been built, it takes the same first/second-declension endings as a regular superlative. Nor do the patterns normally affect the comparative: pulchrior and facilior are regular comparatives.

Adjectives in -er: add -rimus to the nominative

For an adjective in -er, add -rimus, -rima, -rimum to the masculine nominative singular. Because that nominative already ends in r, the result contains -rr-.

The rule uses the nominative, not the shorter stem seen elsewhere. Thus pulcher gives pulcherrimus, not ❌ pulchrissimus; ācer gives ācerrimus, not ❌ ācrissimus. This is a narrow morphological rule, not permission to add -errimus to any adjective whose stem happens to contain r.

Neuter accusative pulcherrimum agrees with templum; masculine nominative plural ācerrimī agrees with hostēs. The doubled consonant belongs to the superlative stem, while the final ending expresses agreement.

The closed -illimus set

Exactly six common -ilis adjectives regularly use -illimus in Classical Latin. Remove the final -is and add -limus, -lima, -limum, producing doubled ll.

This list really must be memorized. Most other adjectives in -ilis take regular -ilissimus: docilis—docilissimus “most teachable,” ūtilis—ūtilissimus “most useful.” English gives no clue about which Latin suffix is correct.

Both neuter nominative superlatives are predicates of infinitive clauses. They do not agree with accusative aliōs or neuter accusative plural vitia.

Feminine nominative simillima agrees with fīlia. Dative mātrī is the complement of similis; it is not the adjective’s head.

Feminine ablative humillimā agrees with casā. The intensive sense “very humble” does not identify a comparison group.

Declension after formation

Both exceptional suffixes produce first/second-declension adjectives. A compact set of cells shows the agreement endings; the same endings attach to ācerrim-, facillim-, and every other member of the two classes.

Both superlatives are feminine accusative singular because both refer to viam. Facillimam illustrates the special suffix; tūtissimam shows the regular suffix in the same syntactic cell.

Feminine accusative plural gracillimās agrees with columnās. The ending -ās, not the doubled ll, marks that feature bundle.

Attributive and predicative placement

These superlatives may be attributive, predicative, or substantival. Position can make the exceptional degree prominent, but it does not change the noun on which agreement depends.

Feminine nominative difficillima agrees with pars, not with nearby neuter genitive itineris. The same relation survives if the predicate is fronted:

The second order makes “most difficult” the frame or focus and sounds more marked, especially in (formal) or (literary) prose. It does not authorize neuter ❌ difficillimum.

Feminine nominative simillima agrees with avis. Dative feminine vērae is substantival—“to a real [bird]”—and completes simillima.

Marginal and historical forms

The productive Classical rules are -errimus after -er and the six-member -illimus list. A few lexical facts sit at their edges. Vetus, veteris has veterrimus, historically connected with an older veter form, while its usual comparative is vetustior. Mātūrus normally has regular mātūrissimus; rare mātūrrimus occurs (literary) and is not a productive model. Such forms must be learned as lexical survivals.

The doubled rr and ll reflect historical assimilation in the superlative suffix. That history explains the shape, but it does not let a learner predict every surviving lexical member. The arbitrary part is real: memorize the six -illimus adjectives, and check a dictionary when an uncommon adjective has competing or defective comparison.

Parsing and production diagnostics

1. If a superlative contains -errim-, test whether its positive masculine nominative ends in -er.
2. If it contains -illim-, test it against the six-item list.
3. Recover the superlative stem, then parse the final first/second-declension ending separately.
4. Locate the head by gender, number, case, and sense; a complement of similis or a nearby genitive is not the head.
5. Keep unusual forms such as veterrimus lexical and rare alternatives register-labeled.

Common Mistakes

1. Forming the superlative from the oblique stem of an -er adjective

2. Extending -illimus to every adjective in -ilis

3. Matching the adjective to a nearby complement

4. Using the exceptional suffix in the comparative too

Key Takeaways

Adjectives in -er normally form superlatives by adding -rimus to the masculine nominative, giving -errimus. Six memorized adjectives in -ilis use -illimus; most other -ilis adjectives remain regular. The comparative stays regular, and every completed superlative declines like bonus, bona, bonum.