Complex sentence: More often the transepts will extend well beyond the sides of the rest of the building, forming the shape of a cross; this is called a "Latin cross" groundplan, and these extensions are known as the arms of the transept.
Split into simpler sentences: More often the transepts extended well beyond the sides of the rest of the building, forming the shape of a cross. This design is called a "Latin cross" ground plan and these extensions are known as the arms of the transept.