helper vs second string

helper

noun
  • That which helps; anything serving to assist. 

  • One who helps; an aide; assistant; auxiliary. 

  • A locomotive that assists a train, usually on steep gradients. 

  • A person who does cleaning and cooking in a family home, or in a market; domestic employee. 

second string

noun
  • A unit of players which is considered to have a secondary level of talent and which usually plays after the first string. 

adj
  • Not as good; of a lower quality or condition. 

How often have the words helper and second string occurred in a corpus of books? (source: Google Ngram Viewer )