Chao (Thai: เจ้า) is a title of the Lannathai royal family members.
Chao Har Kan
Chao Har Kan (เจ้าห้าขัน, lit: Five lords) is the style for the high rank royalty in each Lannathai states.
After the fall of the absolute monarchy, the Chao har kan system was abolished by the People's Party (Khana Ratsadon - คณะราษฎร). In present days, all of the "Chao" family used "na Chiangmai", "na Lamphun" and "na Lampang" as their surname.
References
Chao (monarchy) Wikipedia(Text) CC BY-SA