This article is about the Class III road numbered C636 maintained by DfI Roads. For other roads numbered C636, see C636.
The C636 runs from Stewartstown through Castlecaulfield via Newmills and Donaghmore, although for part of this description it is pretty much two separate routes. From Stewartstown the route leaves the B520 and heads south west, soon splitting at a fork. The northern segment heads west before turning south and south west as it passes by Roughan Lough on the way into Newmills where it meets the C638. Meanwhile the southern segment heads south west, soon crossing the Coalisland branch of the C638. The road maintains its south westward journey as it TOTSOs on the way to a junction with the mainline C638 at Farlough. The route merges together again as it turns off of the C638 in Newmills and heads south west, soon crossing the A29. The road reaches the B43 at Donaghmore and continues off to the south west from a junction further into the village. Passing through Castlecaulfield, the route ends when it reaches the C684 just to the south west. The section of road from Donaghmore through Castlecaulfield was initially numbered as the B121 in the 1920s before being downgraded to class III status.