What is singular and plural form of radii?

Here's the breakdown of the singular and plural forms of "radii":

* Singular: radius (pronounced RAY-dee-us)

* Plural: radii (pronounced RAY-dee-eye)

Why the unusual plural form?

"Radius" comes from the Latin word "radius," meaning "ray" or "spoke." In Latin, the plural of "radius" is "radii," which is why we use the same form in English.