## Key Concepts

### Introduction

LEAN has dozens of methods to create, update, and cancel orders. You can place orders automatically with helper methods or manually through methods on the algorithm API. You can fetch, update, and cancel manual orders with order tickets. As the state of your orders change, LEAN creates events that notify your algorithm.

In backtesting, LEAN simulates order fills with historical data, but you can create your own fill, fee, slippage, and margin models via plugin points. You control how optimistic or pessimistic order fills are with transaction model classes. For more information about these types of models, see Reality Modeling.

### Orders and Tickets

When you create an order with one of the order methods, LEAN creates Order and OrderTicket objects. The order ticket is sent to your brokerage. As the brokerage processes your order, it returns another order ticket and it's compared against the order to see if the order is satisfied. Orders are asynchronous in live trading, so if you want to change an order, you must request it with the order ticket. Order changes are not guaranteed since your order may fill by the time brokerage receives the request.

### Order Types

LEAN supports the following order types:

### Symbol Properties

The SymbolProperties are a property of the Security object. LEAN uses some of the SymbolProperties to prevent invalid orders, and to calculate order quantities for a given target.

SymbolProperties objects have the following properties:

To get the SymbolProperties, use the property on the Security object.

var symbolProperties = Securities["BTCUSD"].SymbolProperties;
var lotSize = symbolProperties.LotSize;
var minimumOrderSize = symbolProperties.MinimumOrderSize;
var minimumPriceVariation = symbolProperties.MinimumPriceVariation;
symbol_properties = self.Securities["BTCUSD"].SymbolProperties
lot_size = symbol_properties.LotSize
minimum_order_size = symbol_properties.MinimumOrderSize
minimum_price_variation = symbol_properties.MinimumPriceVariation

LEAN uses the MinimumPriceVariation to round the LimitPrice, StopPrice, and the TriggerPrice.

Reality models make backtests as realistic as possible to how the strategy would perform in live trading.

If a split event occurred before an order is filled, the unfilled portion of the order will be adjusted automatically, where its quantity will be multiplied by the split factor and the limit/stop price (if any) will be divided by the split factor.