Dú gǔshū de shíhou, chángcháng huì yùdào gǔzì hé tōngjiǎzì.
Reading old books, you often come across old character forms and phonetic loans.
Tips
usage
X 是 Y 的古字 'X is the old form of Y' means Y was made later, usually by adding a radical to X: 云 'cloud' got the rain radical 雨 on top and became 雲. Simplification brought some old forms back, and 云 is the standard character again.
mistakes
Not the same as 繁体字: a traditional character is the unsimplified form still used in Taiwan and Hong Kong, while a 古字 can be older than both forms.
