# Translations:The unit tangent vector/5/en

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At this juncture, we want to clear up one confusing aspect of the mathematics of rays. The usual convention in the case of a raypath is to measure the angle of the tangent line from the vertical. This usage goes right back to the original seventeenth-century statement of Snell’s law. However, in mathematics, the convention is to measure the angle of the tangent line from the horizontal. It is better to stick with conventional mathematics and then convert to raypath usage when the results are displayed. A raypath is a curve along which the seismic energy travels. The unit tangent vector can be written as