Are sideburns part of beard?
Does the beard start at the jawbone or does it start at the above bone?
Can you trim sideburns?
In fatawa mahmoodiya, Hazrat Mufti Mahmmodul Hasan Sahib rahmatullahi alaiyah has written under the definition of the word لُحیٰ (arabic for beard) that it means the bone on which the teeth grow. This comprises of the entire jawbone. This particular bone starts from the slightly protruding bone visible in front of the ear. The hair of the beard grow on this bone, hence, it will be impermissible (haram) to cut the hairs on this bone. Anything above this (protruding bone) is considered part of the head and anything below is considered part of the beard.
To conclude, it is not permissible to trim the sideburns, as they are part of the beard. However, Hazrat Mufti Mahmmodul Hasan Sahib rahmatullahi alaiyah does give concession on cleaning the hairs on the cheek, but he also mentions that the scholars have said it is makrooh to do so. Allamah Anwar Shah Kashmiri rahmatullahi alaiyah has given two reasons why cleaning the hairs on the cheeks, whether it is through shaving with a blade or plucking (threading), is makrooh:
Finally, Hazrat Mufti Mahmmodul Hasan Sahib rahmatullahi alaiyah says that to clean the hair of the throat is completely impermissible. (Fatwa Mahmoodiya page 396-420 volme 19).
Student of Darul Ifta. Checked and approved by Shaykh Abdur Raheem Sahib.