[Thread of execution and the blocking thread] User-level threads can still be parallelized by being scheduled or multiplexed onto different system threads; an example of such scheduling is the M:N model used for Go goroutines, also called hybrid threading. M refers to the number of user threads while N is the number of system threads. Alternative models are 1:1, which means using system threads as the unit of concurrency, and N:1, which means using a single system thread. A notable example of the 1:1 model is the Apache HTTP server, while JavaScript is a notable example of the N:1 model.