What is the unmentionable Italian instrument in Tristram Shandy?

Question

In Laurence Sterne's Tristram Shandy (Volume 1, Chapter XXIII) Shandy is discussing - if I am not mistaken - how people's character may be inferred from external appearances.

Some, for instance, draw all their characters with wind-instruments.—Virgil takes notice of that way in the affair of Dido and Æneas;—but it is as fallacious as the breath of fame;—and, moreover, bespeaks a narrow genius. I am not ignorant that the Italians pretend to a mathematical exactness in their designations of one particular sort of character among them, from the forte or piano of a certain wind-instrument they use,—which they say is infallible.—I dare not mention the name of the instrument in this place;—’tis sufficient we have it amongst us,—but never think of making a drawing by it;—this is ænigmatical, and intended to be so, at least ad populum:—And therefore, I beg, Madam, when you come here, that you read on as fast as you can, and never stop to make any inquiry about it.

What is the unmentionable instrument?

Answer 1

The paper cited in Rand al Thor's comment does indeed deal with the passage that puzzles Mikado. The paper's author, Deborah M. Vlock, suggests that there is a "sexual innuendo" attached to wind instruments generally. She is not specific, but I suspect the innuendo arises from a similarity between the playing of most wind instruments and the act of fellatio. Her other possible sources for the naughtiness of the unnamed instrument with which Sterne teases his reader are more far-fetched.

The sexual innuendo in the wind instrument serves a variety of purposes, including but not limited to the licentious. Sterne also uses it to suggest, in the association of penis and voice, the Italian castrato, a figure who dominated the opera scene through the first half of the eighteenth century. Furthermore, "wind instrument" refers metonymically to the collective orchestral or operatic voice. The passage implicitly criticizes the reliance, by composers of Italian opera seria (who employed castrati), on conventional formulae for their instrumental and vocal representations of the passions, condemning the pretense to "mathematical exactness in their designations of ... character." By the time Sterne commenced writing Tristram Shandy, this "pretense" was common; Handel, the primary operatic composer of the day, relied heavily on Affektenlehre in his operas, and so did his followers.

With no supporting evidence, but going by Sterne’s mention of “mathematical exactness,” the fact that the instrument on his mind can range from piano to forte, and that the instrument’s name is unmentionable, I will hazard a guess—and it is only a guess—that the instrument Sterne refers to is the organ.