Verb-second orders are only found in the Middle Welsh period: Old and Modern Welsh mainly exhibit verb-initial patterns. In this paper I show how the V2 orders developed by carefully reconstructing their syntactic history from earlier patterns with hanging topics and focussed cleft constructions in Old Welsh and related Celtic languages. I provide a syntactic reconstruction of the V2 structures with preverbal functional particles a and y. These C-particles played a pivotal role in relative clauses as well and can be traced back to pronominal elements in Proto-British, the predecessor of Welsh, Breton and Cornish (cf. Schrijver (1997)). I argue that these relative particles in the C-head are the result of Spec-to-Head reanalysis of pronominal phrases and that a similar reanalysis of the adverbial phrase *ed ‘thus’ yielded CVSO orders. During the next stage, the relative clauses in clefts were reanalysed as matrix clauses with V2 order and a process of rebracketing integrated hanging and dislocated topics followed by CVSO into the matrix CP with V2 order as well. These developments resulted in the extension of IS functions for the sentence-initial constituents (beyond original contrastive focus) and a generalised Edge Feature on the C-head.