This is a question that comes up fairly often when looking at the horn parts in Haydn symphonies, because Haydn was notoriously unspecific as to which transposition he wanted.  Sometimes, the answers seem fairly clear cut – for example, if playing in B-flat alto would put the 1st horn in unison with the 1st trumpet all of the time, chances are that Haydn meant B-flat basso – but in other situations things are not so cut and dried. For an explanation of the problem and one possible solution, here’s an extended quote from H.C. Robbins Landon‘s The Symphonies of Joseph Haydn, NY: The Macmillan Company, 1956, p. 124-125.  I’ve put his suggestions to performers in boldface at the end of the quote.

One particular aspect of Haydn’s writing for the horns deserves attention, viz. the problem of horns in B flat. We have seen that Haydn used two types of horns in C, alto and basso. It appears that he also used B flat horns, in alto and basso, but, unlike Mozart, who was very careful to mark his parts ‘high’ or ‘low,’ not one single Haydn autograph specifies which type of B flat horn is to be used, nor do the contemporary parts usually go beyond Haydn’s typical nomenclature of ‘Corni in b Fa’. One MS., that of Symphony No. 35 in the monastery of Stams, and a few later prints, such as Simrock, add alto; otherwise we are left without any inkling of Haydn’s practice.

The fact that Haydn never bothered to write alto or basso over his B flat horn parts leads one to the inevitable conclusion that only one type of instrument was in common use; the question remains: which of the two was meant? An examination of the parts in question suggests that all the earlier symphonies, up to and including the ‘Paris’ symphonies, used alto horns, while as soon as trumpets and drums were used in B flat – which was never the case before the ‘Salomon’ symphonies – the horns dropped to basso. That it was as a rule alto horns which the eighteenth century understood by the term ‘Corni in b Fa’ is obvious from a glimpse at Beethoven’s scores, which continually specify ‘Corni in B basso…[Robbins Landon goes on to detail  more evidence to support his conclusion] Haydn must have begun using B flat basso horns during the London sojourns, as the big C major Symphony No. 97 apparently uses horns in C basso instead of the usual C alto found in his C major symphonies as late as 1788 (No. 90). It is impossible to perform Symphony No. 102 in B flat with alto horns; the technique shows quite clearly that basso is intended (cf. the end of the trio, where horn I enters on written g”). Whether No. 98 is to be performed with alto or basso horns is, however, not so clear; for the horn (and trumpet) writing admits either possibility. It will be safe to perform all the B flat symphonies before No. 98 with alto, Nos. 98 and 102 with basso horns. From this point onwards, especially in the late masses, it is the latter which must be used.(p. 124-125)

The passage ends with the following information concerning Haydn’s Creation. On a personal note, I’ve performed the work twice before, and in the editions we used the B flat horn parts were notated as alto or basso, following the practice Robbins Landon describes below.

An interesting tradition regarding The Creation (1798) has survived, that the arias in B flat major are to be executed with alto horns, the big chorus employing trumpets and drums with basso. Examination of Haydn’s own parts showed no trace of this differentiation; but as the above designations seem to have been added in pencil early in the nineteenth century, it may be assumed that the tradition is an authentic one, going back to a practice which, in Haydn’s day, must have been considered so obvious that no one bothered to write it down. Still, it is to be hoped that more concrete facts relating to this problem will be discovered. (p. 125)

Though Robbins Landon makes a compelling and logical case, to my knowledge there has still been no definitive proof discovered to confirm his conclusion. Paul R. Bryan notes the impact that Robbins Landon’s conclusion concerning alto/basso notation has had on scholars and performers. This passage is from Bryan’s article “Mozart’s Use of Horns in B-flat and the Question of Alto-Basso in the Eighteenth Century,” Historic Brass Society Journal, Volume 14, 2002, p. 165-192.

Twenty-five years ago I published an article comparing the use of horns by Wolfgang Amadeus Mozart and Joseph Haydn [“The Horn in the Works of Mozart and Haydn: Some Observations and Comparisons,” Haydn Jahrbuch IX (1975): 189-255.] Among the more specialized aspects addressed was the question of high (alto) versus low (basso) horns in B-flat. It had previously been raised by H.C. Robbins Landon, who had expressed his opinion that in Haydn’s early symphonies all B-flat horns were high, i.e., alto. He believed that Haydn’s pre-London-period horn parts in B-flat should, therefore, always be performed in the upper octave, a step below written pitch, rather than, as commonly accepted, a ninth below. Subsequently, Landon added “alto” to all the B-flat horn parts that he edited and published in such important series as the complete symphonies of Haydn – as well as those of other composers. My judgment at that time was that in such situations an editor’s opinion might be stated, but that only the composer’s designation should be published in the score.

Landon’s contention, the acceptance of which markedly impacts the sound of Haydn’s orchestra, also influenced the judgments and performances of other scholars and performers. Since 1955 his opinion has gradually been accepted even by a number of specialists on Mozart, with the result that editions of several works in the Neue Mozart-Ausgabe (NMA), as well as many performances and recordings of Mozart’s compositions, have been affected. (p. 165)

One other great resource on Haydn’s horn writing is a website by John MacDonald. His page “A Listing of the Degree of Difficulty of the Horn Parts in the Symphonies of F.J.Haydn,” is full of excellent information.  I’ll close this post with one other story concerning the B-flat alto/basso dilemma, the work in question this time being Haydn’s Symphonie concertante in B-flat major, Hob. I: 105.  We performed this work last season in the Monroe Symphony Orchestra, and the B-flat horn parts were not marked alto or basso. According to H.C. Robbins Landon’s conclusion, this work, which is also scored for B-flat trumpets and timpani, should be performed in B-flat basso.  We performed it that way, and everything worked out fine, but I was surprised to find this recording of the work on YouTube,  performed by the Slovak Chamber Orchestra, Bohdan Warchal, conductor.  To my ears it sounds like there are no trumpets, and the horns are playing in B-flat alto.  Perhaps there is another edition of the work which doesn’t include trumpets?