Zi Collection, Page Position: Lower
Radical: Ten (shí)
Entry: 卅
Kangxi strokes: 4
Page 156, Entry 01
Collected Rimes (Jiyun) and Rime Meetings (Yunhui) define the pronunciation as sa (falling tone); Correct Rimes (Zhengyun) defines the pronunciation as sa (falling tone).
According to Explaining Graphs and Analyzing Characters (Shuowen), it represents three tens. It is now written as the character for thirty, which is ten combined.
According to Broad Rimes (Guangyun), it refers directly to the character for thirty.