第～
だい～ · dai～ · JLPT N2 · noun
thursday
第 + number = ordinal: 第一 first, 第二 second, 第三 third
Put 第 directly before a counting number to make it an ordinal. 第五 (dai-go) means fifth and 第十 (dai-jū) means tenth. It does not change the number's reading, only its position in a sequence.
第 — bamboo radical 竹 (⺮) + 弟 'younger brother'
Originally written with 弟, which depicted a cord wound in order around a stake — implying sequence or order. The bamboo radical 竹 was later added (because bamboo strips were used for numbered tallies and documents), giving 第 the meaning of ordinal number, sequence, or rank.