“pay when paid”和“pay if paid”是工程分包合同中常见的背靠背条款。

“pay when paid”指支付后付款，一般可直接译为背对背条款。这一条款规定上级承包商对下级分包商的支付义务仅在上级承包商获得工程款项后触发。通常情况下，这一条款虽然不会明确表示总承包商收到工程款项是向下级分包商支付工程款的先决条件，但会明确说明相关支付义务的触发要素。这一条款主要是为了加强对上级承包商的保护。

PWP clauses establish the required payment timeliness of upstream contractor's payment obligation to pay lower-tiered subcontractors is triggered only after the upstream contractor is paid. The trigger for obligation will typically have timing provisions associated with the clauses. PWP clauses normally lack explicit language requiring the Prime/GC's receipt of payment as a condition precedent for the payment of the relevant lower-tiered subcontractor's work. The underlying theory behind PWP clauses is to establish and/or increase protections for higher-tiered contractors against claims by lower-tiered subs. PWP clauses normally have explicit "timing provisions" which outline the relevant payment trigger(s). 

“pay if paid”指以获得支付为付款条件，也可直接译为背对背条款，指上级承包商向下级分包商付款的合同付款时间，视上级承包商收到其上级承包商或业主就该下级承包商的作业进行相应全额付款的时间而定。

PIP describes a clause in which the upstream contractor's (usually the Prime/GC) payment obligation(s) to pay downstream subcontractors' claims is/are triggered ONLY, if and after, the receipt of payment, in full, from a higher-tiered contractor or Owner, for the lower-tiered subs' work. PIP clauses are customarily viewed as HIGHLY disfavored and EXTREMELY inequitable. The obligation of Prime/GC to pay downstream subs is triggered ONLY after the Prime/GC's receipt of payment, in full, from Owner for the relevant work billed to Owner from Prime/GC, for work which included the work performed by the lower-tiered subs.

我国工程实践中，对于“pay-if-paid”和“pay-when-paid”条款并未做区分，而统一译为“背靠背条款”，并且法律法规中对此没有作出规定。但住建部印发的《建设工程施工专业分包合同（示范文本）》（GF-2003-0213）第19.5约定“分包合同价款与总包合同相应部分价款无任何连带关系。”

英国此前将“pay if paid”条款和“pay when paid”条款视为等同。但在2009年通过的《地方民主、经济发展和建设法令》第142条[确定应付款]对上述法令该条作了修正，在上述（a）段后增加“施工合同不应以下述内容作为支付的前提条件：①根据另一个合同履行的义务；②任何人关于是否已经履行另一个合同义务做出的决策。”可以看出，英国对该类条款持否定的态度。澳大利亚、新西兰、新加坡和马来西亚等国家的现行法律都规定了禁止总承包商与分包商在分包合同中约定“pay-if-paid”和“pay-when-paid”等类型条款。