It means that the tenor line is an octave lower than treble-clef notation usually is.

That's very common-shifting from two to four staves (with the arrows indicating that score expansion) which necessitates the use of the tenor clef. An upward fourth is an easy reward. ;-)

Amateur singers don't do that well, it's true. But it's a "mistake" in writing found in virtually all polyphony.
Also, what's the issue about the parallel 4ths?

Also, what piece is that? It looks like it's published by GIA (judging by the shape of the vocal brace).

This is called Behold the Lamb of God, Beckenhorst Press Inc, 1993. Written by Craig Courtney (ASCAP)

Behold the Lamb of God

Don,

You will also find the 8 on top of the treble clef, which indicates it is to be played an octave higher (used mainly for piccolos), and the 8 under the bass indicating it is to be played an octave (used for Cellos & Double Basses) The Alto Clef is also called Viola clef because Viola music is written in it.

1) Only 'cellists, bassonists, trombonists, and a few other -ists read Tenor clef anymore. (I have actually seen vocal music that had the tenor clef positioned not on the fourth line, but on the third SPACE, that way the lines and spaces read the same as on treble; I have also seen where the treble clef has a bit of the tenor clef super imposed on it around the third SPACE.)

2) Ledger Lines: tenor parts often extend up to F, G, or even A: this would have too many ledger lines if written in bass clef. So, because of (1), they decided to do this treble clef, octave lower thing, to prevent needing so many ledger lines all the time.

Don't forget that until fairly recently (1900's) vocal music was usually written in the old 'vocal clefs': Soprano, MezzoSoprano, Alto, Tenor, Baritone (all 'C' clefs) and Bass ('F' clef). NB: also until about 1800, even Bass and treble clef were actually movable: you'd often see the bass clef on the Fifth line, rather than the Fourth, for low-lying Bass parts.

Take a gander at this score: Madrigal: "Ride la primavera" by Heinrih Schutz: http://petrucci.mus.auth.gr/imglnks/usimg/d/da/IMSLP75074-PMLP150642-Schutz_Ride_la_primavera_Op1_SWV7.pdf

That score was published in 1890!

The addition of an 8 to a clef for octave transposition is a relatively recent innovation. Formerly, the G-clef was used for tenor vocal and transposing band instruments, such as euphonium (baritone horns), trombones, bass clarinet and saxophones of all types. Why? Well, the fingering of all saxophones is the same, except for transposition, as is the fingering of all clarinets, and a baritone horn had the same fingering of a trumpet except for sounding down an octave. I played many piccolo parts notated with the treble G-clef (without an 8 above the clef) and it was understood that the sound was an octave higher than written.

Perhaps the widespread use of G-clef (adorned or unadorned with an 8) is due to the fact that school children (at least) in the U.S. tended to learn to read the treble clef, at least at first, and the difficulties of negotiating the F- and C-clefs were thereby subverted. I have many scores of oratorio & opera scores (all my Schirmer editions with piano reductions) and other choral works where the tenor parts are written with the unadorned G-clef, with it being understood that the tenor part is sung down an octave from what is written. I'm not sure just when I first encountered the "8" as an added feature on clefs ... perhaps it was because of the advent of computerized engraving programs?

[rantmode]
What gets me perplexed is why so many of those who ardently promote the Doh (C) clef with four-line staves in chant notation have such an apathy towards and aversion to the classical C clef when used in five-liine contemporary notation. Just as the Doh clef can appear on any of several lines, so can the C clef. Or is it the case that chant advocates who can read any Doh clef and transpose at will are just too "something" to give a hoot about the C clef in modern notation?
[/rantmode]

Okay, here is the same choral parts (SATB) with the Tenor notated in Bass F clef, rather then the Tenor G(8vb) clef, The pitches in Don9of11's example just above are exactly the same.

Does that make it clear now?

Actually, Chuck, it's interesting that you mention the aversion to the C Clef. I had never been taught to read it, and was never really forced to learn...until I learned chant, then I had to read the Do Clef--Chant in the Fa Clef was easier at first, since I already knew Bass Clef. After learning the Do Clef in Chant, the C Clef ceased to be an issue.

Though it can be a pain in the neck if you're stuck sight reading a figured Bass part that keeps switching from Bass clef (full Basses) to Tenor Clef ('Cellos and organ only) to Soprano Clef (Organ alone) and back again--When Mozart starts sounding like Schoenberg, it usually means that the continuo player is reading in the wrong clef!

Also, what's the issue about the parallel 4ths?

Parallel 4ths are OK as long as the lower of the pair is not the bass voice (for a planing chord, there ought to be a 3rd under the parallel).

What gets me perplexed is why so many of those who ardently promote the Doh (C) clef with four-line staves in chant notation have such an apathy towards and aversion to the classical C clef when used in five-liine contemporary notation

Because it's "contemporary notation" and doesn't allow them to feel so special and anointed?

Tenors reading treble clef will sound an octave lower than written. The small 8 beneath is a reminder of that.

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