The octave-treble clef (treble clef with a small 8 below) is the standard choral notation for the tenor voice

In choral (SATB) writing, the tenor voice is conventionally notated in treble clef with a small '8' below the clef sign — indicating the part sounds an octave lower than written — rather than in true tenor (C) clef or true bass clef. This 'octave-treble' or 'treble-8' clef lets sopranos, altos, and tenors all read from visually similar treble-clef notation while the tenor's SOUNDING pitch sits correctly an octave below what is written, which is the near-universal convention in choral octavo publishing.

A four-part chorale exercise notating the tenor voice in plain treble clef with no octave indication (sounding a full octave too high if literally read as written) or in true bass clef (which is the OTHER common tenor notation but changes which ledger lines the part uses and is a genuinely different, also-valid choice worth distinguishing from the treble-8 error case).

Gould, Behind Bars, Clefs chapter and Vocal Music chapter both state the treble-8 convention for tenor voice as standard choral practice. Ross, The Art of Music Engraving, agrees and notes it as close to universal in commercially published choral scores. Read, Music Notation, agrees. This is one of the more universally-agreed conventions in the whole chapter.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.