We open 1m and respond 1M. Except when m=♦ and M=♥, there is a "tweener"
bid at the 3 level which isn't used for anything. The three possible auctions
are:
1♣-1♥-3♦ALERT
1♣-1♠-3♦/♥ALERT
1♦-1♠-3♥ALERT
In all three cases, Opener is showing a raise to 3M based on "shape", and
a direct raise to 3M shows specifically 18-19 balanced.