How do shears and scissors differ?

In the United States shears and scissors are often used interchangeably. As a rule, however, shears is employed when the implement is large and scissors when it is small. All such instruments having a total length of six inches or less are called scissors in the hardware trade; those exceeding six inches in length are called shears. In Scotland all sizes are called shears, the word scissors being seldom used. The Oxford dictionary says: “The larger instruments of this kind, especially those which are too large to be manipulated with one hand, are called shears.” Thus we speak of sheep shears and pruning shears. Scissors, since the term is applied to a single implement, is sometimes construed as singular, although it is plural in form. It is correct to say “the scissors are sharp” or “the scissors is sharp/’ tut the former is preferred. The word was. written in the plural form and construed as a plural as early as 1384. Pair of scissors is recorded, as early as the fifteenth century.