Lachmann's law

Lachmann's law is a Latin sound change that lengthened the root vowel of certain past participles before a voiced stop followed by the suffix -tus.

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Latin perfect participle: A Latin participial form that commonly expresses a completed action and agrees with a noun in gender, number, and case. Lachmann's law is most visible in selected perfect participles formed with -tus.
actus: The Latin perfect participle of agere, meaning “driven” or “done,” with a long root vowel. Its long ā precedes -tus where the verb's present stem contains a voiced stop.
Karl Lachmann: A German classical scholar and textual critic whose name is attached to Lachmann's law. The sound change was named for Lachmann.
Laryngeal theory: A theory reconstructing consonants in Proto-Indo-European that later affected neighboring sounds and vowels. Proposed accounts of Lachmann's law consider how inherited consonants and their loss shaped Latin forms.
Latin phonology: The sound system and sound patterns of Latin across its historical stages. The law is a historical phonological pattern in Latin.
captus: The Latin perfect participle of capere, meaning “seized,” with a long root vowel. Its long ā exemplifies lengthening before -tus after the voiced stop in the historical stem.
Proto-Indo-European: The reconstructed ancestor of the Indo-European language family. Comparative evidence from inherited forms helps explain the pre-Latin consonants behind the pattern.
Proto-Indo-European glottalic theory: A proposal that Proto-Indo-European stop consonants included ejective or otherwise glottalic sounds. Alternative reconstructions of the ancestral stop system bear on explanations of the law's conditioning.
Voiced stop: A consonant made by briefly blocking airflow while vocal-fold vibration continues. The conditioning consonants in Lachmann's law are voiced stops before the participial suffix.
rectus: The Latin perfect participle of regere, meaning “directed” or “ruled,” with a long root vowel. Its long ē is a familiar example associated with Lachmann's law.