Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE Performance ( bandmate VARCHAR )
TABLE: CREATE TABLE Band ( lastname VARCHAR, id VARCHAR )

### Question ###
What is the last name of the musician that has been at the back position the most?

### Accurate SQL ###

SELECT T2.lastname FROM Performance AS T1 JOIN Band AS T2 ON T1.bandmate = T2.id WHERE stageposition = "back" GROUP BY lastname ORDER BY COUNT(*) DESC LIMIT 1