68th Street Hunter College subway station, New York City, New York

I can't accurately describe the sound of three guitars hitting the door of a subway train, but guitars are cheap so you can bang one with a spoon three times and there you have it.

Three short Mexicans with short-necked guitars hustled themselves into the train and held the doors for a taller Mexican with a double bass. The tall man wore a moustache; all wore straw cowboy hats with brims folded up for manouevrability in the throng. They could turn their heads without pecking.

Before the doors had closed the four-piece had struck up: a folk song in Spanish, almost ludicrously quick, the beat annoyingly close to twice as fast as the rhythmic motions of the train. Two guitarists sang with dry, beautiful untrained voices as the bass player chewed his moustache and plucked.

The bass strings were painted with red, white and blue stripes which didn't line up string to string but probably once had, and the paint was coming off where the strings hit the black wooden neck of the bass.

I put down Francis Parkman, Jr's The Oregon Trail and looked at the third guitarist. He was taking off his hat to use it as a cup and then he slung the guitar behind him, dropped a few quarters into it to make a rattle and walked down the train, rattling his hat in time to the music. I pulled out a $10 and a single and gave him the single rolled up into a ball. The man sitting to my right gave him $5 which was all he pulled out of his pocket.

The train pulled in to 59th Street station just as the band were finishing a verse. Without slowing they grew louder over a couple of seconds and then finished up with a yell! 'Gracias, amigos', the oldest Mexican said as he finger-flourished his guitar, and the four were gone, out of the train and back in, into the next carriage.

I heard the three guitars bump into each other as they made way again for the bass. Why did the third guitarist bother to tune his guitar?