Blazars are a special class of AGN, with relativistic jets pointing towards the observer's line of sight. They emit in almost all wavelengths ranging from radio to very high energy gamma-ray. The complex nature of multi-wavelength (MW) emission of blazars can be explained by studying the MW SED. To model the MW SED, generally, people use the single zone emission model. However, sometimes the single-zone emission model fails to explain the MW emission. In this talk, I will discuss the conditions where the single emission zone is not suitable to describe the MW emission. Further, I will discuss the particular case of PKS 1510-089, whre a two-zone emission model is required to explain the total SED.
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