Chinese · Pronunciation Guide
yee-dz: a low sagging first syllable, then a weak toneless second. The two i's in the pinyin represent completely different sounds.
yǐzi
Dip then nothing. The neutral suffix insulates yǐ from anything on the right, so it keeps its full descent even when a dipping syllable follows the word. On the left it is fully exposed, and since chairs are usually counted, the measure word in front of it is often a dipping tone too.
Listen: 椅子
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这把椅子很舒服。
Zhè bǎ yǐzi hěn shūfu.
This chair is comfortable.
Listen: 这把椅子很舒服。
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这把椅子很舒服。 shows that exposure: bǎ and yǐ are adjacent dipping tones, so bǎ rises. Then zi fades, hěn keeps its dip because shū is level, and shūfu ends with a level syllable and a toneless one. Two dipping tones in the sentence, one converted and one not.
The two i spellings are the trap: yǐ is a genuine "ee" and zi has no vowel at all, and learners pronounce them alike. Say the word slowly and notice that the tongue moves from high-front to teeth-forward. The other error is giving zi a dipping tone by analogy with 子 as an independent character — it is weightless here.