Adjectives in ας, αινα, αν, and in ης, ες, add the τερος and τατος to the neuter, as—

μέλας,black,μελάντερος,-τατος.
ἀληθής,true,ἀληθέστερος,-τατος.
ἀκριβής,accurate,ἀκριβέστερος,-τατος;

a termination which is assumed also by adjectives in ων, as εὐδαίμων, happy, εὐδαιμονέστερος,-τατος.

Some adjectives are compared by ιων,-ον, and ιστος,-η,-ον, as—

καλός,beautiful;
καλλίων,more beautiful;
κάλλιστος,very beautiful.
ἡδύς,sweet, ἡδίων, ἥδιστος.
ταχύς,swift;
θάσσων for ταχίων, swifter;
and τάχιστος,very swift.

This form belongs also to some common irregular comparisons as—

ἀγαθός, βελτίων, βέλτιστος,good,better,best.
μέγας,great,μείζων,μέγιστος.
κακός,bad,χείρων,κάκιστος.
πολύς,many,πλείων,πλεῖστος;

which follow in declension the law of the substantives of the same type, as μείζων, μείζονος; πλείων,-ονος, plural πλείονες, contracted πλείους.

Comparatives are either accompanied with a genitive or followed by ἤ—as

ὁ υἱὸς μείζων ἐστὶ τοῦ πατρός,  the son is taller than the father