In cases where j survived between vowels (specifically after long vowels), it expanded into ij to form a diphthong with the preceding vowel. Tolkien described this development in the Outline of Phonetic Development (OP1) from the 1940s:

After long vowels y, w remained and w was strengthened to TQ v. So māya, rāwa [rāva]. In the case of māya there was a tendency to mā̆iya, often represented in spelling maia, maiya (OP1: PE19/62).

In addition to Q. Maia, examples include:

• ✶áyan > Q. aian “holy thing” (PE17/149).
• ✶lājā > Q. laia, present tense of lá- “to not be” (PE22/153).
• ✶mōjă > Q. moia- “labour, be afflicted” (VT43/31).

This change did not apply to ēya or ūya however:

• Q. néya “once” (VT49/31).
• Q. Aldúya “Day of the Two Trees” (LotR/1110).
• Q. núya- “descend” (PE22/163).

Tolkien sometimes gave variant forms with both aiya and aia, such as:

• ᴹ✶wāyā > ᴹQ. vaia or vaiya “envelope” (Ety/WAY).

See also the rule whereby [ij], [uw] became [i], [u]

Conceptual Development: In Qenya Phonology of the 1910s Tolkien gave numerous examples of áı̯-, óı̯-, úı̯- > aiy-, oiy-, uiy- (PE12/12), but here the accent indicates stress rather than length, and the numerous variant forms make it difficult to puzzle out Tolkien’s intent.

@@@ PE19/62