FORTUNE -- In a note to clients issued Monday afternoon, 24 hours before Apple (AAPL) is scheduled to release its earnings for the March quarter, Piper Jaffray's Gene Munster talked about something we don't hear much about: The so-called buy side.
Buy side is Wall Street jargon for those investing institutions such as mutual funds, pension funds and insurance companies that tend to buy large portions of securities for money-management purposes. In other words, the Big Money.
Sell side analysts -- also known as equity research analysts -- are employees of the brokerage firms that sell stocks to the buy side. This is the class of Wall Street professionals that includes Munster and the rest of analysts whose estimates, price targets and recommendations (buy, sell, hold etc.) I collect each quarter.
Munster goes even further. Not only does his employer make a market in Apple -- buying and selling the stock -- but Munster himself has an investment banking relationship with the company.
But back to his Monday note. In addition to offering his estimates for Apple's March quarter and his best guesses for what the Street expects (guesses that align quite closely with mine and are copied below), Munster reports the results of a survey of buy side investors he conducted over the past two weeks.
The gaps between the buy and sell sides in terms of iPhone unit sales and company gross margins, as Munster estimates them, are represented in the chart above. As I say, this is not something you usually hear much about. But it may help explain how market is likely to react after Apple reports its earnings tomorrow. If Apple comes in below the buy side's expectations, the stock could take another beating.
Below: Munster's sell-side spreadsheet.
CORRECTION: An earlier version of this story misstated Munster's relationship with Apple. He is not paid to do market research for the company.
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