A word should be said on Oughtred's definition of + and -. He recognizes their double function in algebra by saying (Clavis, 1631, p. 2): "Signum additionis, sive affirmationis, est + plus" and "Signum subductionis, sive negationis est - minus." They are symbols which indicate the quality of numbers in some instances and operations of addition or subtraction in other instances. In the 1694 edition of the Clavis, thirty-four years after the death of Oughtred, these symbols are defined as signifying operations only, but are actually used to signify the quality of numbers as well. In this respect the 1694 edition marks a recrudescence.
The characteristic in the Clavis that is most striking to a modern reader is the total absence of indexes or exponents. There is much discussion in the leading treatises of the latter part of the sixteenth and the early part of the seventeenth century on the theory of indexes, but the modern exponential notation, a^n, is of later date. The modern notation, for positive integral exponents, first appears in Descartes' Géométrie, 1637; fractional and negative exponents were first used in the modern form by Sir Isaac Newton, in his announcement of the binomial formula, in a letter written in 1676. This total absence of our modern exponential notation in Oughtred's Clavis gives it a strange aspect. Like Vieta, Oughtred uses ordinarily the capital letters, A, B, C, . . . . to designate given numbers; A^2 is written Aq, A^3 is written Ac; for A^4, A^5, A^6 he has, respectively, Aqq, Aqc, Acc. Only on rare occasions, usually when some parallelism in notation is aimed at, does he use small letters[27] to represent numbers or magnitudes. Powers of binomials or polynomials are marked by prefixing the capital letters Q (for square), C (for cube), QQ (for the fourth power), QC (for the fifth power), etc.
Oughtred does not express aggregation by (). Parentheses had been used by Girard, and by Clavius as early as 1609,[28] but did not come into general use in mathematical language until the time of Leibniz and the Bernoullis. Oughtred indicates aggregation by writing a colon (:) at both ends. Thus, Q:A-E: means with him (A-E)^2. Similarly, { root}q:A+E: means { root}(A+E). The two dots at the end are frequently omitted when the part affected includes all the terms of the polynomial to the end. Thus, C:A+B-E=.. means (A+B-E)^3=.. There are still further departures from this notation, but they occur so seldom that we incline to the interpretation that they are simply printer's errors. For proportion Oughtred uses the symbol (::). The proportion a:b=c:d appears in his notation a·b::c·d. Apparently, a proportion was not fully recognized in this day as being the expression of an equality of ratios. That probably explains why he did not use = here as in the notation of ordinary equations. Yet Oughtred must have been very close to the interpretation of a proportion as an equality; for he says in his Elementi decimi Euclidis declaratio, "proportio, sive ratio aequalis ::" That he introduced this extra symbol when the one for equality was sufficient is a misfortune. Simplicity demands that no unnecessary symbols be introduced. However, Oughtred's symbolism is certainly superior to those which preceded. Consider the notation of Clavius.[29] He wrote 20:60=4:x, x=12, thus: "20·60·4? fiunt 12." The insufficiency of such a notation in the more involved expressions frequently arising in algebra is readily seen. Hence Oughtred's notation (::) was early adopted by English mathematicians. It was used by John Wallis at Oxford, by Samuel Foster at Gresham College, by James Gregory of Edinburgh, by the translators into English of Rahn's algebra, and by many other early writers. Oughtred has been credited generally with the introduction of St. Andrew's cross x as the symbol for multiplication in the Clavis of 1631. We have discovered that this symbol, or rather the letter x which closely resembles it, occurs as the sign of multiplication thirteen years earlier in an anonymous "Appendix to the Logarithmes, shewing the practise of the Calculation of Triangles etc." to Edward Wright's translation of John Napier's Descriptio, published in 1618.[30] Later we shall give our reasons for believing that Oughtred is the author of that "Appendix." The x has survived as a symbol of multiplication.