Tenor

Tenor Scale
This is a scale length of 406mm (16in) to 460mm (18in) and these days usually about 430mm (17in)

The history of where the 17in median length for this scale came from is easy to trace, but where the name came from and why Tenor, are much tougher questions. The 17in scale length was, like the 15in Concert, a Martin construct, (though I should say here this is also the same sort of scale length as the Rajao, one of the precursor instruments to the Ukulele, but I don't think this had any bearing on the actual origins). At the end of the 'teens William J Smith & Co., a major distributor of Martin products at the time, asked Martin to produce a kind of Chordophone based around the Colombian Tiple. As a result of this request, in 1919 Martin produced a 17in scale 10 string Ukulele that became the Tiple Ukulele.

For all of the tuning "fun" involved with it, the instrument proved popular and Martin continued production of it. 9 years later in 1928 Martin used this body size and scale length to start production of a new, large 4 string Ukulele, tuned g~C~E~A, and they called this their Tenor, (This may have been to counter Gibsons new TU Ukulele that was also this size; though it is more likely Gibson would have copied Martin?). Though the two most prestige US manufacturers were making them, at the time the scale length didn't catch on that much, none of the other big manufacturers introduced them, possibly because they were sticking with the Standard Approved smaller sized definition of Tenor?) and only a few of the more specialist Ukulele firms had a go before WWII.

Post WWII it was only really Soprano and Baritone scales that regained popularity in the US. However both Martin and Gibson continued production of their Tenors throughout this period and Kamaka started to make them too. Because of this 17in became the de facto Tenor scale length. With the decline in Ukulele popularity again in the late 1960's even Martin and Gibson gave up production of all Ukuleles, leaving only Kamaka in Hawaii as a serious manufacturer; but by this time Kamaka had taken all of the Martin scale lengths, (before the war they only made Sopranos), as their own.

With the Ukulele rising in popularity again post millennium, it was the old Martins, (Gibsons to a lesser extent), and the output of Kamaka that gave the basic definitions to what scale lengths should be, and the larger 17in scale proved popular with Guitarists moving down to the Ukulele, thus bringing it to far more prominence now that it ever had historically.

Reason for the Name
The origins of, and reasons behind the name itself are, as I have said, a bit more of a mystery. I am not aware of it appearing in any makers catalogue before 1926 and the National Association of Musical Instrument and Accessories Manufacturers proclamation of "official" Standard Approved scale definitions and string lengths so it may be that they came up with it? But why, (and this goes for whoever came up with it), when the other two sizes were called Standard and Concert, was the switch made to a musical term? Possibly because the prevalent Ukulele tuning for the smaller instruments at the time was a~D~F#~B and this larger instrument always had a lower g~C~E~A tuning? Whatever it was this name, and the later introduction of the Baritone, led to the Standard becoming a Soprano, (but the Concert never became an Alto?). That is my guess and if it was for another reason I don't know it.

Tunings
The standard tuning for a Tenor is g~C~E~A and with Nylon/Gut strings. It never goes higher, though there are some people who do like to go lower, even down to e~A~D#~F#; this is not particularly common though. What is quite common as a tuning alternative though is to not have reentrant tuning and have the G an octave lower as the lowest note, (this is called Low G tuning and does require a different, thicker or heavier string for the G)'. This moves the Ukulele away from its traditional sound and the "wall of sound" style of playing but gives a greater range of tone particularly for single string solo playing or finger picking. It has also led to the development of a 5 string Ukulele where the G has become a two string course with both a high and low G to try and give the best of both worlds. The tuning can be complicated further by the use of strings designed for a Baritone D~G~B~E tuning, (or reentrant d~G~B~E) but this takes the instrument into the Jumbo Tenor / Mini Baritone arena which can be a quagmire of understanding as different people use different terms to describe the same thing! This Wiki's definition is:- If the Ukulele has a scale length of 460mm or less but is tuned D~G~B~E, (reentrant or not), it is a Mini Baritone. if it has a scale length of 461mm or more but is tuned G~C~E~A, (reentrant or not), it is a Jumbo Tenor; but this has no true authority.

The Build
Tenors are almost always built with a "waisted" figure 8 body; it is more common now to see the neck of a Tenor join the body at the 14th fret, (higher that the 14th and the body size usually starts to shrink making it a Super Concert or Super Soprano), but they started out joining at the 12th and some people still make them that way.

The Tenor is probably the most popular scale length for solid body electric Ukuleles and these are often made to look like miniature versions of solid body electric Guitars. It is also the case that these days that the Tenor scale is the most popular scale length for Taropatchs and Lili'us

It is very unusual, but not unheard of to see a Pineapple body shaped Tenor scale Ukulele, or any other of the body shapes you can often find with Sopranos, (Round body, Teardrop, Paddle and the like), but there are some people who make highly inventive custom shaped Tenors.