On March 5, 1574, English mathematician and Anglican minister William Oughtred was born. After John Napier invented logarithms, and Edmund Gunter created the logarithmic scales (lines, or rules) upon which slide rules are based, it was Oughtred who first used two such scales sliding by one another to perform direct multiplication and division; and he is credited as the inventor of the slide rule in 1622.
William Oughtred attended the presticous Eton School and enrolled at King’s College Cambridge in 1592. He received his M.A. eight years later and was especially interested in mathematics, even though the field has not been taught extensively at Cambridge back then. Outhred became Episcopal minister in 1603 and became vicar of Shalford one year later. Oughtred was announced rector of Albury in 1610 and took mainly private pupils who received a mathematical education.
In 1631, Oughtred published his most impirtant work, ‘Clavis Mathematicae’. In it, he described Hindu-Arabic notations and decimal fractions as well as algebra. Oughtred began to experiment with new mathematical symbols like x for multiplication and :: for proportions. He used π for the circumference and introduced ‘greater than and less than’ symbols, which were not accepted by the community due to being too hard to remember. The now very familiar < and > were introduced by Harriot. 
William Oughtred became especailly famous for an early version of the slide rule. Already in 1620, the English clergyman, mathematician, geometer and astronomer Edmund Gunter managed to plot a logarithmic scale along a single straight two foot long ruler. He began to add and substract lengths by using a pair of dividers. The operations used were equivalent to multiplying and dividing. William Oughtred invented a circular slide rule about one decade later. In 1632, Oughtred published Circles of Proportion and the Horizontal Instrument describing slide rules and sundials. 
It is believed that William Oughtred and Richard Delamain invented the circular slide rule independently from each other. Delamain published his Grammelogia, or the Mathematical ring already in 1630. Big discussions arose over the topic and throughout Oughtred’s later life. 
At yovisto you can learn more about “Why is ‘x’ the symbol for an unknown?” in a humorous talk by Terry Moore.
References and Further Readings: