Mendelssohn and the Organ Sonata

I should like to call the pieces “3 Sonatas for the Organ” instead of “Voluntaries”. 
Tell me if you like this title as well, if not, 
I think “Voluntaries” will suit the pieces also, the more so as I do not know what 
it means precisely. 

Felix Mendelssohn wrote these lines to Charles Coventry in a letter dated 29 August 1844. Coventry was the proprietor of the English music publisher Coventry & Hollier and a great admirer of his friend Mendelssohn. As background for their correspondence in late summer of 1844, Mendelssohn had been commissioned to write organ music by Coventry & Hollier, organ music intended for the English market. The term voluntary is used for a piece of organ music bearing a certain resemblance to the classical French overture, with a slow followed by a fast movement.