Lieutenant Commander T'Pang in 2415
Lieutenant Commander Rank Insignia
The Starfleet line rank immediately inferior to Commander, and superior to Lieutenant. Officers with this rank typically hold positions as senior department heads on larger vessels such as the Galaxy and Sovereign class. A Lieutenant Commander will typically serve as the executive officer of smaller or older vessels. On larger starships, the second officer holds this rank (such as Data of the U.S.S. Enterprise or Tuvok of the U.S.S. Voyager). Officers of this rank may be addressed informally as simply "Commander".
Officers of this rank typically have anywhere between 5-15 years of experience in Starfleet.
When introducing a Lieutenant Commander, it is proper to introduce them using their full rank. However, it is polite to address them (and refer to them) as "Commander." For an inferior officer to address a Lieutenant Commander as "Lieutenant" is considered a breach of decorum. It is risky for inferior officers to address them as "Lieutenant Commander", aswell. Some may take offense; others may not care.