The second most-common failing in most men's clothing is the simple 
error of showing up for an event either over- or under-dressed. The 
former is harder to do than the latter, but still awkward -- a tuxedo 
will stand out anywhere that isn't black-tie, and a matched suit looks 
strange outside of formal business settings or particularly dressy 
social occasions. Conversely, wearing casual trousers and an informal, 
patterned jacket or no jacket at all can be a serious disadvantage in 
high-stakes (and high-formality) dealings. 
The
 first and easiest way to be sure of wearing the right thing is to 
follow the dress code, if one is offered -- most social occasions will 
include this information in an invitation. Understanding the basic 
terminology makes wardrobe choices significantly easier:
White Tie
Rarely seen on modern invitations, white tie is the most formal of 
dress codes. It includes tailcoats, piped trousers, and white 
waistcoats, and is prohibitively expensive for most men. Unless you 
happen to attend jet-set dinners or work for very large charities, 
you're unlikely to ever face the white tie code.
Black Tie
Black tie or evening dress means that a black or midnight blue dinner 
jacket and matching trousers is expected. A silk bow tie is the only 
appropriate neck wear (matched to the lapel facings) and patent leather 
pumps or highly-polished
Oxfords are theonly shoes that should be worn. The shirt should be white, French-cuffed, and fastened with studs.
Black Tie Optional
Black tie optional is frequently used for ceremonies where the 
participants will be formally dressed, but want to spare their guests the
 necessity of owning or renting a tuxedo. A solid, dark suit with a dark
 tie and a white undershirt is perfectly appropriate at these events, but
 anything patterned is too informal. Once again, shoes should be 
unadorned black Oxfords.
Semi-Formal or Business Dress
Semi-formal or business dress should not be confused with casual or 
business-casual; a suit is still expected. The fabric should be dark and
 patterning kept to a minimum, and the shirt should be unobtrusive and 
light-colored. A tie is necessary, as are simple, conservative shoes and
 belt.
Business-Casual or Dress-Casual
Business-casual or dress-casual implies that a tie in particular is 
optional, and in some circles also indicates that a jacket can be 
omitted or replaced with a sweater, vest, or similar garment. The shirt 
must still be collared, and dress trousers are expected. Any leather dress shoe is appropriate.
Casual
Casual is not anything-goes; any event that bothered to provide a dress
 code still expects attendees to look neat and well-presented. However, 
jeans and casual shirts including polo or golf shirts are appropriate, 
and dress shorts may also be considered within bounds in hot weather. 
Leather sandals, moccasins, or boat shoes may be worn.
When no 
specific code is given, it may be best to err on the side of 
conservative dress and wear clothing that can be adjusted necessary -- a
 suit that proves to be too formal can be dressed down by removing the 
tie, or the jacket of a casual outfit can be set aside to leave you with
 a simple collared shirt and trousers.
When more casual codes are 
in place, resist the temptation to throw on an old polo and a pair of 
khakis; the result will be looking like every other schlub in the room. 
Practice the art of dressing up while dressing down instead: wear light,
 loose sportcoats with active patterns and softer colors to look casual 
without losing the flattering shape of a well-cut jacket. In the summer 
heat, lightweight materials can keep trousers and long-sleeved shirts a 
viable option -- and there's always the American classic of the 
seersucker suit, the very epitome of Southern casual.
Overall, 
it's easy to avoid being the wrong-dressed man if you keep your wardrobe
 versatile enough to conform to different dress codes and pay attention 
to expectations at public events. And in general, remember that it's 
always safer to be overdressed than under, since clothes can be removed 
but not added (without a trip home, at least).