A normal slide rule is used to perform multiplications and divisions and find the value of functions like sine, cosine and tangent, reciprocal and exponential. Its invention goes back to the seventeenth century, but it was in the mid twentieth century that they were most popular and many specialised forms with extra scales were developed. Multiplication and division are performed by adding or subtracting  the logarithms of the numbers involved by sliding the logarithmic scales past each other. Although circular slide rules have always been available, linear forms have been most popular.