Sen Sōshitsu (千宗室) is the traditional name carried by the head of the Urasenke family.
He is generally known as Sensō Sōshitsu (仙叟宗室), without mention of the family name, and is counted as the fourth generation in the Urasenke family line.
The current head of Urasenke is the sixteenth generation, Sen Sōshitsu XVI, who is distinguished by his cognomen, Zabōsai.
The kanji character for sō, 宗, in the hereditary name may be interpreted to mean "family core".
For example, the head of the Omotesenke school traditionally carries the name Sōsa, written 宗左, and the head of the Mushakōjisenke school is Sōshu, 宗守.