Cartoon of the multi-stranded loop. The loop with diameter D is composed of many individual strands with diameter d. When observing the loop, each spatial resolution element of size R of the instrument corresponds to a column of the cross section. The hashed area represents one such column. Strands that are bright in a particular EUV channel (or line) are indicated by a back dot, and empty circles represent strands not radiating in this channel (at this instant in time). This example consists of N_t = 79 strands in total, of which N_b = 25 are bright, corresponding to a fraction f_b = 0.3. For the loops on the Sun we estimate that they consist of N_t ≈ 2500 strands. See Sect. 3.3.