Radical 5 meaning "second" is one of six of the 214 Kangxi radicals that are composed of only one stroke. In the ancient Chinese cyclic character numeral system tiāngān, 乙 represents the second Celestial stem.
Fazzioli, Edoardo (1987). Chinese calligraphy : from pictograph to ideogram : the history of 214 essential Chinese/Japanese characters. calligraphy by Rebecca Hon Ko. New York: Abbeville Press. ISBN0-89659-774-1.