6a85405c_mental__Scottish_Formal_Attire__jacket
[  ]   matching waistcoat, low cut and fastened with three Celtic buttons Wikipedia also states that the waistcoat can be tartan (as shown in the Lowland Dress photo below) but that option seems to apply only to the Argyll jacket or Sheriffmuir doublet [jacket] 
waist covering
k i l t
w a i s t c o v e r i n g
s h i r t
n e c k w e a r
f o o t w e a r
a c c e s s o r i e s
o u t e r w e a r
Black Tie Guide | Supplemental: Scottish Formal Attire
jacket