Thomas Morley Describing a Hypothetical Competition in Improvised Singing (1597)
‘When I learned descant  from my master Bold he, seeing me so inclined and willing to learn, always had me in his company, and because he always carried a plainsong book in his pocket, he caused me do the same. And so, walking in the fields, he would sing the plainsong and cause me to sing the descant and when I sang not to his contentment he would show me where I had erred.
There was also another descanter, a companion of my master, who never came in my master’s company (although they talked together often) but they fell to contention, striving to see who should bring in the point  the soonest, and who could make the hardest proportions, so that they thought they had won great glory if they had brought in a point sooner, or sung harder proportions than the other:
But it was a world to hear them argue, each one defending his own [descant] for the best:
- What? You don’t keep time in your proportions
- You sing them false, what proportion is this?
- No, you don’t know what you’ sing, it seems that you have recently come from a barbershop , where you had Gregory Walker , or a Curranta played in the new proportions recently discovered by them, called sesquiblinda and sesqui harken after 
so that if one unacquainted with music had stood in a corner and heard them, he would would have sworn that they were out of their wits, so earnestly did they fight over such a trivial thing.
In truth, I myself sometime thought that they would have started fighting about the matter, for the descant books were made Angels , but yet fists were not visiters of ears, and therefore all parted friends: but to tell the truth, this Poliphemus had a very good sight, (especially for treble descant) but a very bad utterance, so that his voice was the worst that I had ever heard, and although others thought him very good in that respect, nobody thought better of him than he did of himself.
If one had asked his opinion of the best composers living at this time, he would say in a vain glory of his own sufficiency, “tush, tush,” for these were his usual words, “he is a proper man, but he is no descanter; he is no descanter there is no stuff in him, I will not give two pins for him unless he has descant.”’
‘when I learned descant of my maister Bould, hee seeing me so toward and willing to learne, euer had me in his companie, and because he continually carried a plainsong booke in his pocket, hee caused me doe the like, and so walking in the fieldes, he would sing the plainsong, and cause me sing the descant, and when I song not to his contentment, he would shew me wherein I had erred, there was also another descanter, a companion of my maisters, who neuer came in my maisters companie (though they weare much conuersāt together) but they fel to contention, striuing who should bring in the point soonest, and make hardest proportions, so that they thought they had won great glorie if they had brought in a point sooner, or sung harder proportions the one then the other: but it was a worlde to heare them wrangle, euerie one defending his owne for the best. What? (saith the one) you keepe not time in your proportions, you sing them false (saith the other) what proportion is this? (saith hee) Sesquipalterysaith the other, nay (would the other say) you sing you know not what, it should seeme you came latelie from a barbers shop, where you had Gregory Walker, or a Curranta plaide in the newe proportions by them latelie found out, called Sesquiblinda, and Sesqui harken after, so that if one vnacquainted with musicke had stood in a corner and heard them, he would haue sworne they had beene out of their wittes, so earnestlie did they wrangle for a trifle, and in truth I my selfe haue thought sometime that they would haue gone to round buffets with the matter, for the descant bookes were made Angels, but yet fistes were no visiters of eares, and therefore all parted friendes: but to say the very truth, this Poliphemus had a verie good sight, (speciallie for treble descant) but very bad vtterance, for that his voice his voice was the worst that euer I heard, and though of others he were esteemed verie good in that kinde, yet did none thinke better of him then hee did of himselfe, for if one had named and asked his opinion of the best composers liuing at this time, hee woulde say in a vaine glory of his owne sufficiencie, tush, tush (for these were his vsuall wordes) he is a proper man, but he is no descanter, hee is no descanter there is no stuffe in him, I wil not giue two pinnes for him except he hath descant.’
 Descant: Morley gives a few definitions of ‘descant,’ but the context of this passage makes it clear that he means the act of ‘singing a part extempore vpon a playnesong, in which sence we commonly vse it: so that when a man talketh of a Descanter, it must be vnderstood of one that can extempore sing a part vpon a playne song’
 A point: A theme.
 Barbershop: Barbershops often had groups of amateur musicians to entertain the clients.
 Gregory Walker: This was the name of a particular type of 16th-century wig, furthering the connection with barbershop music.
 Sesquipaltery, sesquiblinda and sesqui harken after: These are made up names for proportions.
 Made angels: The books were thrown up in the air.
While carrying a plainsong book around and practicing descant with a friend sounds like a lovely way to spend the afternoon, for those of us without suitably enthusiastic friends, the (free) app ‘Neumz’ provides a very useful tool for practicing descant, with both a rolling score and recordings of chant according to the liturgical calendar.
Although the rhythm is too ‘declamatory’ to practice singing much more than simple counterpoint, this in itself provides an extra challenge and develops reflexes.
Thomas Morley, A Plain and Easy Introduction to Practical Music (London: Peter Short, 1597). My paraphrase made for ease of reading.
The image below is Morley’s first example of how to ‘maintain a point’ when singing (improvising) descant