Prior to 1934, the heir system to the throne of Perak was much longer. Have a look at the chart above and you can see the now non-existent titles of Raja Di-Hulu, Raja Di-Darat and Raja Di-Baruh, which were of higher ranking than the Raja Kechils.
[*] In 1934, the Adat Lembaga was established by Sultan Iskandar Shah to streamline the lineage.
[**] In 1948, Sultan Yusuf Izzuddin revised the lineage during his reign. The Raja Kechil Sulong rank replaced Raja Kechil Besar, whilst the ranks of Raja Kechil Muda and Tengah were swapped, making RKM the successor to the rank of RKS.
[***] In 1953, the Constitution of Perak, Second Part, 1953, section XXXVII (1), (2), (3) and (4) was established, thus – “33. Extinction of the title of Raja Bendahara. The title of Bakal Sultan now known as Raja Bendahara shall lapse and be extinguished on the present holder ceasing to hold the title, in which event the Bakal Sultan now known by the title of Raja di-Hilir shall be the next in rank to the Raja Muda: Provided that on the lapse and extinction of the said title of Raja Bendahara, the title of Orang Kaya Bendahara Sri Maharaja shall be resuscitated.”