Constrained skills theory (Paris, 2005) conceptualizes reading development as a progression or continuum of two spheres of literacy skills. Constrained skills are finite (e.g., 26 letter names; 44 phonemes), can be mastered in a relatively short window of time, and are easy to assess. These pre-decoding and "word recognition" skills include concepts of print, phonological and phonemic awareness, phonics, decoding, spelling, sight word recognition, and morphology. Name writing, for example, is a highly constrained activity; most learners accomplish it between the ages of three and five.
Unconstrained skills or "meaning-based" skills, on the other hand, are never fully mastered. These large domains of knowledge are more challenging to teach and to quantify because they are acquired gradually and continue to develop incrementally over a lifetime. Unconstrained skills are also more strongly predicted by children's socio-economic status and parents' education. Vocabulary, background knowledge, and comprehension are unconstrained skills that typically require higher level critical thinking abilities.
Skilled reading requires both constrained and unconstrained abilities. If a would-be reader lacks or has limited "word recognition" and/or "meaning-based" skills, s/he will struggle.