Casing Connections Types | API & Premium
Casing Thread Connections

Oilfield tubular may be equipped with plain ends (no threads), have API-specified threaded casing thread connections, or proprietary (non-API) threaded connections.

API Casing Connections 

Oilfield casing conforming to API standards may be obtained with plain ends, but ends

are usually threaded and furnished with couplings such as:

- short thread and coupling (STC)
- long thread and coupling (LTC)
- buttress thread and coupling  (BTC)
- Extreme-Line thread (
https://www.drillingmanual.com/oil-gas-casing-design-casing-connections/