Many opponents of the American spelling - most notably Marcus Campbell of Paddy Not Patty fame - like to point out that Patty is widely used as the shortening of Patricia, or in reference to the circular piece of beef that you place inside a bun, while Paddy is short for Pádraig, the Irish name for Patrick.
Despite the online uproar that this annual debate typically produces, however, there seems to be little in the way of a consensus as to why, in fact, Americans do substitute the Ds for Ts. One belief is that America - a nation rich in Irish heritage - does not have Paddy in its proper noun lexicon. It would be like people in the United Kingdom struggling over a name like Barack (some say buh-rok, others say ba-rak). Another belief is that the obvious shortening of Patrick would be Patty, since both words are made up of Ts and are totally devoid of Ds.
Another reason - and one that people seem to overlook - is that Americans, on the whole, pronounce their double Ts as if they were double Ds. So words like better become bedder, and bottle becomes boddle etc. It would not be difficult to imagine, upon first hearing the phrase St. Paddy's Day, that some Americans might have mistaken it for St. Patty's Day, and thus the latter eventually stuck.
Whatever the reason, when the 17th of March arrives, tradition holds that so much alcohol is consumed that revelers - be they Irish or American - are so drunk by the day's end that spelling their own name becomes a challenge.
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