I play 2/1 with 1♣ open show 3+ cards.
I play walsh responses to 1♣ open.
Responder with less than opening hand with ♦ and M usually hide his ♦ suit and bid 1M.
Responder with opening hand with ♦ and M tend to bid his ♦ suit and intend to bid his M suit.
Suppose bidding was 1♣-(Pass)-1♦-(1M)-dbl
What this dbl show ?
Plus... it's free!