Consider following score of Monteverdi's Sì dolce è 'l tormento for soprano and continuo (SV 332) from IMSLP. What is the meaning of the capital letters above the soprano part? They do not seem to correspond to chord names.
The letters correspond to particular tablature of the 5-sting guitar popular at the time. Here's one tablature translation from: http://www.patriciadixon.net/guitar-lit-html/italianbaroque.htm