The word "桅杆" is formed by compounding two phono-semantic characters. "桅" combines the semantic radical 木 (wood) with the phonetic component 危 (wéi) to denote a mast. "杆" combines the semantic radical 木 (wood) with the phonetic component 干 (gǎn) to denote a pole. Together, they form a compound word where both characters contribute their meanings to specify a wooden mast or flagpole.