can be covered by the casing trim system 12 to provide a cased wall opening 18 the casing trim system 12 can comprise a pair of side or jamb trim pieces 22 and a head or transom trim piece 24 which can be attached to the trimmers 33 and the header plate 32 respectively e g by nailing with shims 25 installed therebetween for proper positioning side and top casing trim pieces 26 27 are applied to the jamb and head trim pieces 22 24 and to the trimmers 33 and the header plate 32 through the panels 30 the casing trim pieces 26 27 thus overlap the panels 30 slightly to conceal the unfinished edges thereof the tool 10 generally comprises a base 40 a pair of clamp assemblies 42 and a level 44 the base 40 includes first and second ends 45 46 and a rectangular cross sectional configuration the base 40 further includes an outer contact or trim engagement face 37 and an inner face 38 the base 40 has a pair of side edges 49 and a plurality of nailing holes 43 each clamp assembly 42 is mounted on the base inner face 38 in proximity to a respective end 45 46 and includes first and second clamp brackets 52 a b each bracket 52 a b includes a respective base leg 54 a b mounted on the base inner face 38 and projecting laterally from a respective base side edge 49 each bracket 52 a b also includes an extension leg 56 a b extending outwardly from a respective base leg 54 a b each first clamp bracket 52 a includes a toggle clamp 58 with an over center.