second string vs understudy

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. 

understudy

verb
  • To act as an understudy (to someone). 

  • To study or know a role to such an extent as to be able to replace the normal performer when required. 

  • To act in a similar manner to some known person. 

noun
  • A performer who understudies; a standby. 

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