間

Here's an explanation about the relationship between the characters 間, 閒, and 閑:

These characters are 会意字 (compound ideographs). Originally, 間 was a variant form of 閒 (meaning "leisurely" or "free"). 閒 itself is composed of 門 (gate) and 月 (moon), symbolizing "moonlight coming through the gate," which metaphorically means "interval," "space," or "leisure."

The character 間 replaces the 月 (moon) in 閒 with 日 (sun/day), resulting in a character with the same meaning related to "space," "interval," or "time." Historically, many traditional dictionaries, including the Kangxi Dictionary (강희자전), treated 間 simply as a variant or subsidiary form of 閒 and often did not list it separately or explained it in detail.

However, despite this traditional treatment, 間 became much more widely used than 閒 over time. In modern usage, 間 is officially recognized as the standard form and has largely replaced 閒.

On the other hand, 閒, which means "leisurely" or "free," has sometimes been interchanged with 閑, another character meaning "leisurely." Nowadays, 閒 itself is often treated as a variant of 閑, which has made the status of 閒 somewhat ambiguous.

As for usage, these characters can appear at the end of words or alone to express meanings such as "during," "while," or "period of time."