The great majority of the letters are to Andrew Fletcher, Lord Milton; there are a few scroll letters by him and a few to other members of the family. Under each year the letters are arranged by the relationship of the writer to Lord Milton: his parents Henry Fletcher and Margaret Carnegie; his siblings Robert, Mary (Hepburn), and Martha; his wife Elizabeth Kinloch ('B.K.'); his children Andrew, Henry, John, Francis, Archibald, Margaret (Grant; 'Peg'), Elizabeth (Wedderburn), and Mary ('Mally'); and his niece Margaret Hepburn. More distant relations are grouped among the general correspondence (MSS.16525-16733).

In the first half of the period the subjects are chiefly family and estate business; then the education of the children comes into the picture; and finally the careers of the children become prominent, especially Andrew as Member of Parliament (regular reports on politics from 1747) and Henry in the Army (in the Low Countries and America during the Six Years' War).