Sometimes, the easiest way to determine what you should wear for a semi formal dress code is by first figuring out what you shouldnt wear
Buy Go Mai Women's Church Dress Suit for Ladies - Mother's Gifts, Wedding Party, Formal Church Clothes and other Suit Sets at Amazon.com

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Given that the latter is more casual than the former, and we dont want to repeat ourselves any more than is strictly necessary, well plump here for plain, dark selvedge jeans with minimal bells, whistles and western pocket detail
Neck Size to Dress Shirt Size Conversion This chart maps your neck measurement to standard dress shirt sizes and approximate suit jacket sizes