Triad vs diad of muscular tissue 

T tubule also known as transverse tubule is substructure of skeletal muscle .

It is formed by deep invagination of sacrolemma of skeletal muscle and two sets of terminal  of sarcoplasmic reticulum, So one tubule and two sacroplasmic reticulum, these three structure formed triad of skeletal muscle. 

Each skeletal muscle cells contain thousands of triad . T tubules are place perpendicular to skeletal muscle fibers . 

Triad place at AI junction of sarcomere of skeletal muscle 

it form anatomical basis of excitation contraction coupling, 

A stimulation start from neuromuscular junction, pass through the T tubule, calcium release from sarcoplasmic reticulum 

in cardiac muscle T tubule contain only one terminal cisterna so known as diad