The voiceless dental spirant mainly developed from the primitive aspirate [tʰ] (PE19/18), but medially after vowels it was voiced to [ð] (PE19/23), and thus [θ] from primitive [tʰ] appears more often initially than medially.

Another way that [θ] could develop was from the spirantalization of [t] after liquids and voiceless stops, and thus [θ] could appear non-finally in clusters such as [rθ] or [lθ] (“rth” or “lth”). In the case of pairs of primitive voiceless stops, these often reduced to a single [θ] because double spirants became single. In addition, in the Doriathrin dialect at least, a final [ð] was again devoiced to [θ], giving another way for a [θ] to appear non-initially.