The old national standard is round head (R), countersunk head (F) and half countersunk head (O), the new national standard has pan head (P), countersunk head (F) and half countersunk head (O), American machine screws have various head types, in addition to the commonly used pan head, countersunk head, half countersunk head and large flat head, hexagonal washer head, Warwick head, etc.. Countersunk head is also called flat head, and semi-countersunk head can also be called umbrella head. One-way slot is also called slotted
