Interesting. I'm mostly a fiddler so I was thinking of instrumental laments and slow airs, not of songs. I wonder if there are musical (i.e., non-lyrical) attributes that can characterize laments. Minor keys, I guess, but I wonder if there are other specific tricks that evoke a sad mood.

And again, with that word pastoral - are there musical attributes that make an instrumental piece a pastoral, or just singing about sheep? Or maybe if the composer chooses a title related to the countryside or wants it known that he was inspired to write it by a trip to the moors, he calls it a pastoral. The "pastoral" instrumental piece in Handel's Messiah is followed by the air beginning "He shall lead his sheep like a shepherd," so maybe that's why.

Is "Sheep May Safely Graze" a song or an instrumental?

Marion