The traditional 樂 is a picture of a musical instrument. The early forms show silk threads (the two 幺 elements) strung across a wooden stand (the 木 at the base), the image of a frame instrument like a zither or a rack of drums. The original meaning is therefore music, read yuè. The simplified 乐 collapses that whole structure into five quick strokes, but the two characters are one and the same word-family, simply written differently: 乐 is the mainland simplification of 樂.

From "music" the character grew a second reading. Because music gives delight, 樂 / 乐 came to also mean joy, happiness, and that sense took the reading lè. So one written sign carries two spoken words:

1. yuè · music. The original sense, the strung instrument: 音乐 yīnyuè (music), 乐器 yuèqì (musical instrument), 乐曲 yuèqǔ (a musical composition).
2. lè · happy, joyful. The derived sense, the gladness music brings: 快乐 kuàilè (happy), 欢乐 huānlè (joyous), 乐观 lèguān (optimistic), 乐意 lèyì (willing, glad to).

The link between them is intuitive: what is music for, if not joy? Read the character as yuè when it names sound and instruments, and as lè when it names the feeling. The thread of delight runs through both, the instrument on one side and the smile it draws on the other.

The lè reading does more than name a mood. It also marks a glad willingness, the disposition of someone who takes pleasure in doing a thing rather than merely agreeing to it. 乐意 lèyì is "to be glad to," 乐于 lèyú is "to take pleasure in" some activity, and 乐趣 lèqù is the delight a pursuit gives. The common thread is that the doing itself is a source of joy, not a chore.

This is why 乐 sits at the heart of phrases about generosity and contentment. To help others 乐 is to find the helping itself pleasant; to be 乐 in one's lot is to be at peace with it. The reading lè turns joy from a passive feeling into an active leaning toward what one enjoys.

When 乐 names sound rather than feeling, it is read yuè, the original reading that recalls the strung instrument of the ancient glyph. The anchor word is 音乐 yīnyuè (music), where 音 yīn supplies "sound, tone" and 乐 yuè supplies the organized art of it. From there the yuè reading runs through the whole vocabulary of music: 乐器 yuèqì (instrument), 乐曲 yuèqǔ (composition), 乐队 yuèduì (band, orchestra), 乐谱 yuèpǔ (sheet music).

The rule of thumb is straightforward. If the word is about sound, instruments, or compositions, the character is yuè. If it is about gladness, willingness, or fun, it is lè. The same five strokes, two spoken words, told apart by what they point at: the music, or the joy it makes.

Hold the traditional 樂 in your mind first: silk threads strung across a wooden stand, an instrument waiting to be played. That picture is the original meaning, music, read yuè. Now play it, and watch the joy rise off the strings. That joy is the second meaning, happiness, read lè.

The reading tells you which face you are looking at. Sound, instruments, and compositions take yuè: 音乐, 乐器, 乐曲. Gladness, willingness, and fun take lè: 快乐, 乐意, 乐趣. One instrument, two gifts. Ask whether the word means the music or the smile, and you will know whether to say yuè or lè.