Here is an example:
```abc
T:Ned Of The Hill
R:waltz
M:3/4
L:1/8
K:Gmaj
%%clef bass
B,,A,,|{F,,}G,,2 E,,3 F,,|G,,4 {B,,}G,,A,,|B,,2 G,3 F,|E,4 E,2|D,4 B,,A,,|G,,4 A,,B,,|C,3 B,, A,,G,,|E,,4 B,,A,,|
```
As I recall, there are extensions to the ABC format, and commands beginning with %%, like %%clef, are some of those. I changed the syntax to
K:Gmaj clef=tenor
and it formatted as expected, even for the somewhat unusual (for anyone but a cellist) tenor clef.
Thanks for the tip!