Two fractions are said to be adjacent if their difference has a unit numerator. For example, 1/3 and 1/4 are adjacent since 1/3 - 1/4 = 1/12, but 1/2 and 1/5 are not since 1/2 - 1/5 = 3/10. Adjacent fractions can be adjacent in a Farey sequence.

Farey sequence | Ford circle | fraction | numerator