Symbol used for Metric thread
M                       ISO Metric Screw Thread

Symbols used for British Threads
BA                      British association standard thread
BSC                    British standard bicycle thread
BSF                    British standard fine thread
BSP                    British standard pipe thread
BSPT                  British standard pipe thread, tapered
BSW                   British standard Whitworth coarse thread
CEI                     British bicycle thread
WHIT                  British Whitworth standard special thread

Symbols used for American Threads 

Acme-C              Acme thread, centralizing
Acme-G              Acme threads, general purpose
Stub Acme          Stub Acme thread
AMO                  American Standard microscope objective thread
N.Butt                National Buttress thread
NC*                   American National coarse thread series
NF*                   American National fine thread series
NEF*                 American National extra fine‑thread series
8N*                   American National 8‑thread series
12N*                 American National 12‑thread series
16N*                 American National 16‑thread series
NH                    American National hose coupling and fire hose coupling
                        threads
NGO                  American National gas outlet threads
NM                    National Miniature threads series
NS*                   Special threads of American National form
NC                    American Standard coarse thread series
NF1                   American Standard fine thread series
NPT                   American Standard taper pipe thread
NIPTF                American Standard taper pipe thread (dryseal)
NPTR                 American Standard taper pipe thread for railing fittings
NPS                   American Standard straight pipe thread ,
NPSC                 American Standard straight pipe thread in couplings
NPSF                 American Standard internal straight pipe thread (dryseal)
NPSI                  American Standard intermediate internal straight pipe
                        thread (dryseal)
NPSM                 American Standard straight pipe thread for mechanical 
                        joints
NPSL                 American Standard straight pipe thread for Locknuts &
                        Locknut pipe thread
NPSH                 American Standard straight pipe thread for hose
                         couplings and nipples
ANPT                 Aeronautical taper pipe thread
RMS,                 American Standard surveying instrument mounting thread