epiphone has two types of LP standards; plain top and plus top. i know that the difference is obviously on the "top" of the guitars, but what exactly is different? the looks? the quality of the wood itself?
it may have something to do with the thickness. A lot of budget guitars have a very thin laminate on the top - the binding on the sides or black finish usually hides this.

Maybe the plus top is thicker?
well, the plain top is unflamed giving it the sorta faded look it has. the plus top is flamed giving that tiger striped looking finish. i think those are the biggest differences.
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plain tops have no flamed maple or quilted maple tops. usually, i think they're 1/4 inch thick, but please correct me if i am wrong.
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