NB: More recent documentation for Dyalog APL is available at https://docs.dyalog.com/20.0/.

Left Tack ⊣
Monadic Left Tack means

Same 

      ⊣  1 2 3
1 2 3
Dyadic Left Tack means

Left 

      'L' ⊣ 'R'
L
      ⊣/ 1 2 3
1

Language Elements