First-string and First class

First-string adjective - Of the very best kind.

First class and first-string are semantically related In some cases you can use "First class" instead an adjective phrase "First-string".

First class noun - Mail that includes letters and postcards and packages sealed against inspection.

First-string and first class are semantically related in better topic. Sometimes you can use "First-string" instead an adjective phrase "First class", if it concerns topics such as superior, excellent, first-rate, highest in importance.