Handel's orchestral works range from the Water Music suites of 1715 and 1717 and the Music for the Royal Fireworks to the five sets of Concerto Grossi, and include 13 single concertos representative of every period of his creative life. The limitation of his "Orchestral Music" to these works of course takes no account of the innumerable overtures, concertos, sonatas, etc, which are included in his operas, oratorios, anthems and other vocal works: indeed the distinction can only be theoretical, for a large number of movements in his five sets of Concerto Grassi and the individual concertos are arrangements or direct borrowings from earlier choral and orchestral works, keyboard and chamber music.
Of the Opus 3 set, Six Concerti Grossi (Oboe Concertos), published by John Walsh in March or April 1734, only the following were original: No. 1 in all three movements; No. 2, 1st, 2nd and 5th movements; No. 3, 3rd movement; No. 5, 3rd movement. Many of these original movements consist merely of a short Adagio linking longer movements derived from other works.
On the other hand his Opus 6 set Twelve Grand Concertos ... (written in the short period of September to October 1739, and published by Walsh in April 1740) consists largely of original work as far as has been ascertained up to the present time. There are, however, oboe parts included in the British Museum autograph R. M. 20. g. 11, in Nos. 1, 2, 5 and 6. The 1st, 2nd and 6th movements of No. 5 are reworkings of the three movements of the overture to the Ode for St. Cecilia's Day first performed on 22nd November 1739, which in turn are indebted for their material to Gottlieb Muffat's Componimenti musicali of the first half of 1739. From this one may suspect that Nos 1, 2 and 6 of Opus 6 may also have had their origin in former compositions.
Of Handel's three sets of Organ Concertos, Opus 4, Nos. 1, 2, 4 and 6 are original (the set of six was published by Walsh in 1738). The second set, without opus number (written c. 1739 and published by Walsh in November 1740) contains hardly any original work, for Nos. 2 to 6 inclusive are transcriptions of Opus 6, Nos. 11, 10, 1, 5 and 6 in that order, whilst No. 1 is derived from Opus 5, No. 6, Sonatas or Trios for two Violins, or German Flutes with ... Harpsichord.., and Opus 6, No. 9.
Handel's third set of Organ Concertos, Opus 7 (composed 1740-50, but not published until 1760), consists almost entirely of original work, the exceptions being No. 1 in B flat major, whose finale is based on a movement in No. 6 of Gottlieb Muffat's harpsichord suites, Componimenti musicali of 1739; and No. 4 in D minor whose second movement derives its ritornello from Telemann's Musique de Table, and whose finale is the same as that of Concerto VI in Opus 3.
The ritornello material of the finale to Opus 3, No. 6, and Opus 7, No. 4 is one of Handel's commonest gambits. This was recognized as far back as the end of the 18th century, for a manuscript of Opus 3, No. 2 in the Fitzwilliam Museum, Cambridge, which can be dated 1750-60, contains the following entry in pencil: In this Minuet (4th movement) we find his favorite passage, as introduced in his first Pastor Fido, Esther, etc. Handel's earliest use of this thematic material can be traced back to 1710-20, and examples of its use can be cited in more than a dozen instrumental works, as well as in several vocal works. Handel uses this motive in a wide variety of tempi.
Handel's skill as an improviser on the harpsichord and organ is well known. His Organ Concertos, designed with himself as soloist for use between the sections of his oratorios, abound in directions such as 'organo ad libitum' (and even, organ adagio e fuga ad libitum'), no organ part being written out or only a mere sketch of a theme. There are strong reasons for believing that many of his finished and polished compositions had their origin in an improvisation at the harpsichord, starting with a theme such as is described above and developing it according to his special genius. One of the secrets of Handel's genius is, however, that it matters not whether the motive comes from himself or another composer -- the resulting movement always bears an indefinable imprint which is unmistakably Handelian.
Comparisons between Handel and Bach seem to be inevitable when discussing the former. In critical writings Bach's strength and variety in the use of harmonic and contrapuntal resources are frequently mentioned, while Handel is upheld for his broadness of effect: such arguments are profitless, as one suspects that they so frequently arise from a false spirit of partisanship. Many passages in Handel suggest that he was capable of harmonic strength and great contrapuntal skill.