< th > = vl vd th

Voiced th: -rth-, VthV

swarthy
prithee
smoothie

Voiceless th: -lth-, -thic, 'th


-lth- : wealthy, healthy, filthy
-thic: gothic, monolithic, 
'th: 'thesis, ca'thartic, ca'thedral, etc (exception: al'though)
-athy: apathy, homeopathy
lengthy


cf. wesly (-sl-)