In order to pronounce an /l/ you have to place the tongue against the lower palate and upper teeth and expel the air through the two channels that remain open on both sides of the tongue (while your vocal cords vibrate to produce voice). That's why we describe the /l/ as a "lateral" sound. To produce an /r/ or /rr/, on the contrary, you have to PREVENT the air from escaping laterally and force it to get out through the CENTRAL intermittent crevice your tongue creates while flapping against the palate with more or less amplitude of movement to yield double or simple /r/, respectively. The /r/ /rr/ is therefore not described as "lateral", but as "central", in that respect.

Just two different letters. :P You're gonna have to be more specific!