Shittan Renjōshū is a work on the rule of renjō (連声) or sandhi, i.e., a range of phonological changes that occur at morpheme or word boundaries, composed by Chōzen (澄禅, 1613-1680), an important Japanese monk of the early Edo period.
Siddham is a script, or rather a group of scripts developed in South Asia around the 5th century CE. The script receives its name from the practice of writing the word siddhaṃ, meaning ‘accomplished’ in Sanskrit, at the head of documents. The script reaches China as a script for writing Buddhist texts in around the 6th and 7th century CE and is later brought to Korea and Japan. The Japanese word shittan (悉曇) refers to the script but is also used to indicate the phonetic study of Sanskrit (shittanshō, 悉曇章). In his Shittan Renjōshū, Chōzen, as other shittan scholars before him, uses the Japanese katakana as well as kanji to mark the pronunciation of Sanskrit alphabets written in the Siddham script. Also note that individual Siddham letters are sometimes arranged vertically in the East Asian context, as they are in this book, because of the influence of Chinese and Japanese which are written vertically in this period.