Second string and Under

Second string noun - A squad of players that are available either individually or as a team to relieve or replace the players who started the game.

Under and second string are semantically related. In some cases you can use "Under" instead an adjective phrase "Second string".

Under adjective - Located below or beneath something else.

Second string and under are semantically related. Sometimes you can replace phrase "Second string" with "Under", this terms are similar.