Start with 52 cards and no jokers. Deal seven columns containing 1, 2, 3, 4, 5, 6 and 7 cards. Only the top card in each column starts face up. The remaining cards form the stock.
Worked example
The first four columns contain 1 + 2 + 3 + 4 = 10 cards. The last three contain 5 + 6 + 7 = 18 cards. Together the columns use 28 cards. Subtract 28 from the 52-card deck to find the stock size.
The Top Card Is Available
S3U uses Draw One: each draw turns one stock card face up onto the waste. Only the top waste card can move. If a 4 of clubs is covered by a 6 of diamonds, the 6 is available and the 4 is not. When stock is empty, turn the waste over without shuffling. S3U permits repeated stock passes. Other solitaire variants use different rules.