Square lashing: The square lashing is used to secure two crossed poles, usually for construction. Start with a clove hitch, and pass the rope around the poles three times (see video), followed by three frapping turns, and secure with two half hitches, NOT a clove hitch as shown in the video. (Source: howtowilderness Youtube Channel)
Round lashing: A round lashing is used to secure two parallel poles, end-to-end, to make a longer pole. You might use this to make a flagpole. Plenty of overlap should be allowed between the ends of the pole so that it doesn’t pivot or twist and fail. If you’re connecting two 2m poles, for example, you might allow 0.5m overlap.
Prusik square lashing for synthetic rope: Used to connect two crossed poles, where slippery synthetic rope is being used. The lashing starts with a Prusik knot before using a square lashing to join the poles. Some sources say the knot should be finished with a Surgeons Knot – a reef knot with an additional turn in the first half knot. See the Prusik Knot and Square Knot videos below.