变调

In Mandarin tone sandhi, the most famous rule is the 三声变调: when two third tones appear in a row, the first becomes a second tone (你好 → ní hǎo). Other rules cover 一 before different tones, and 不 before fourth tone (不是). Pinyin spelling normally still shows the underlying tones.

usage

Same word covers two unrelated 'tone change' meanings - phonological 变调 (linguistic tone sandhi) and musical 变调 (key modulation). Context disambiguates: 汉语变调 = sandhi, 钢琴变调 = key change.