In English CiV Lengthening, non-high lax vowels are not allowed in the __CiV environment when a morpheme boundary intervenes (e.g., Canada: ˈCan[ə]da, Caˈn[eɪ]d-ian, *Caˈn[æ]d-ian). At first blush, CiV Lengthening appears to be a derived environment effect that is blocked from applying in non-derived environments e.g., ˈc[æ]meo). However, CiV Lengthening is similarly blocked in derived environments when the target vowel is not newly stressed (e.g., Orwell: ˈOrˌw[ɛ]ll, ˌOrˈw[ɛ]llian, *ˌOrˈw[i]llian). This paper reports experimental results, which show that CiV Lengthening productively applies to newly stressed stems and is blocked from applying to non-newly stressed stems. A new analysis of CiV Lengthening is then proposed, which recasts CiV Lengthening as a case of the emergence of the unmarked. When the target vowel is a non-reduced vowel (as in ˈOrˌw[ɛ]ll), a high-ranking faithfulness constraint acts to keep tenseness unchanged (as in ˌOrˈw[ɛ]llian). This novel analysis rests on the idea that neutralized vowels like [ə], having lost the tenseness distinction, are faithful neither to tense nor to lax full vowels. Hence, for a word like ˈCan[ə]da, the decision falls to a lower-ranked Stress-to-Weight Principle constraint that picks the candidate with the heavy [eɪ] (i.e., Caˈn[eɪ]d-ian) as the winner. Thus, CiV Lengthening can be modeled via traditional markedness and faithfulness constraints, and does not need the complex constraint machinery often associated with derived environment effects.