The word analogue entered English from French in the 19th century and ultimately traces back to the Greek word logos, meaning "ratio." (The word analogy, which has been a part of English since the 15th century, also descends from logos.) The noun analogue is sometimes spelled analog, particularly when it refers to a chemical compound that is structurally similar to another but slightly different in composition. Adding to the confusion, there is also an adjective spelled analog, which came into use in the 20th century. The adjective can refer to something that is analogous (as in an analog organ), but it is most often used to distinguish analog electronics from digital electronics (as in an analog computer or an analog clock).