Writing Sanskrit in Latin letters requires either diacritics or compromises, and the choice reaches into how the language is understood by people who meet it in translation. The script has three sibilants where the Latin alphabet has one s. It has two full series of stops, one made at the teeth and one with the tongue curled back, where Latin has one t and one d. It has four vowels that carry a syllable on their own with no Latin equivalent at all, and a set of voiced aspirates that Latin writes with an added h in a way that invites exactly the wrong reading.
Where the diacritics are dropped, and outside specialist publishing they usually are, the distinctions disappear. Words that differ in the language become identical on the page, the aspirated dental is read as the sound in the English word think, and the retroflex series vanishes into the dental one. A reader working from that has no way of recovering what was written.
The Siddham letters have none of these problems, because each distinction has a character of its own and no character does double duty. The East Asian tradition made that argument in practice rather than in theory: it transcribed mantras into Chinese characters for approximate reading, and it also kept the Siddham original, because only the second could be read back. This atlas uses plain Latin spellings without diacritics, for readability, and the letter pages are where the distinctions those spellings lose are recorded.